Marker Manager will not show markers

Question

Use a debugger.

for the first value of place (0) place.posn (0.posn) is undefined; it should be stations[place].posn
There is no createMarker function in your code.

<!DOCTYPE html>
<html>
  <head>
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <style type="text/css">
      html { height: 100% }
      body { height: 100%; margin: 0; padding: 0 }
      #map-canvas { height: 100% }
    </style>
    <script type="text/javascript"
      src="https://maps.googleapis.com/maps/api/js?key=AIzaSyBPtpV_saW7qtACfPVYBWIB-Jw253iBca4&sensor=false"></script>
    <script type="text/javascript" src="http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markermanager/src/markermanager.js"></script>
    <script type="text/javascript">
        var stations =[
            { "name": "Oostende", "posn": [51.233, 2.917], "code":"(64080,99999)" },
            { "name": "Gent", "posn": [51.183, 3.817], "code":"(64310,99999)" },
            { "name": "Koksijde", "posn": [51.083, 2.65], "code":"(64000,99999)" },
            { "name": "Wevelgem", "posn": [80.817, 3.2], "code":"(64160,99999)" },
            { "name": "Melle", "posn": [50.983, 3.983], "code":"(64340,99999)" },
            { "name": "Uccle", "posn": [50.8, 4.35], "code":"(64470,99999)" },
            { "name": "Antwerpen", "posn": [51.2, 4.467], "code":"(64500,99999)" },
            { "name": "Retie", "posn": [51.217, 5.033], "code":"(64640,99999)" },
            { "name": "Goetsenhoven", "posn": [50.783, 4.95], "code":"(64630,99999)" },
            { "name": "Schaffen", "posn": [51, 5.067], "code":"(64650,99999)" },
            { "name": "Diepenbeek", "posn": [50.917, 5.45], "code":"(64770,99999)" },
            { "name": "Chievres", "posn": [50.567, 3.833], "code":"(64320,99999)" },
            { "name": "Bierset", "posn": [50.65, 5.45], "code":"(64780,99999)" },
            { "name": "Kleine Brogel", "posn": [51.167, 5.467], "code":"(64790,99999)" },
            { "name": "Ernage", "posn": [50.567, 4.683], "code":"(64590,99999)" }
        ];
        var red_marker = "http://maps.google.com/mapfiles/ms/icons/red-dot.png";
        var green_marker = "http://maps.google.com/mapfiles/ms/icons/green-dot.png";
        var map;
        var mgr;
        var site_marker;
        var markers;
    var infowindow = new google.maps.InfoWindow({});
        function setupMap() {
            var latlng = new google.maps.LatLng(50.85, 4.35);
            var mapElem = document.getElementById("map-canvas-9");
            var mapOptions = {
                zoom: 8,
                center: latlng,
                mapTypeId: google.maps.MapTypeId.ROADMAP
                };
            map = new google.maps.Map(mapElem, mapOptions);
            site_marker = new google.maps.Marker({
                position: latlng,
                map: map,
                title: " latitude N, longitude ",
                icon: red_marker
                });
            var listener = google.maps.event.addListener(map, 'bounds_changed', function(){
                setupNoaaMarkers();
                google.maps.event.removeListener(listener);
            });              
        }
        function setupNoaaMarkers() {
            mgr = new MarkerManager(map);
            google.maps.event.addListener(mgr, 'loaded', function(){
                mgr.addMarkers(getNoaaMarkers(),8);
                mgr.refresh();
            });      
        }
        function getNoaaMarkers() {
            var markers=[];
            for (var place in stations) {
                var title = place.name;
                var posn = new google.maps.LatLng(stations[place].posn[0], stations[place].posn[1]);
                var code = stations[place].code;
                var marker = createMarker(posn, title, code);
            }
            return markers;
        }
        function changeSitePosition() {
            var lat = document.getElementById("id_latitude").value;
            var lon = document.getElementById("id_longitude").value;
            if (lat === 0 & lon===0)
            {
                lat = 50.85;
                lon = 4.35;
            }
            var latlng = new google.maps.LatLng(lat, lon);
            site_marker.setPosition(latlng);
            map.setCenter(latlng);
        }
  function createMarker(latlng,title,code) {
    var marker = new google.maps.Marker({
      map: map,
      title: title,
      position: latlng
    });

    google.maps.event.addListener(marker, 'click', function() {
      infowindow.setContent(code);
      infowindow.open(map, this);
    });
    google.maps.event.addListener(marker, 'click', function() {
      if (document.getElementById(code).checked) {
        document.getElementById(code).checked=false;
      } else {
        document.getElementById(code).checked=true;
      }
    });


    return marker;
  }

    </script>
  </head>
  <body onload="setupMap()">
    <div id="map-canvas-9" style="width: 800px; height: 300px;" class="easy-map-googlemap">
    </div>
    <table>
        <tr>
            <td>Latitude</td>
            <td><input id="id_latitude" max="90" min="-90" name="latitude" step="0.0001" type="number" /></td>
            <td></td>
            <td>In decimal degree, within [-90,90], max. 4 decimals, e.g. : 45.5 </td>
        </tr>
        <tr>
            <td>Longitude</td>
            <td><input id="id_longitude" max="180" min="-180" name="longitude" step="0.0001" type="number" /></td>
            <td></td>
            <td>In decimal degree, within [0,360], max. 4 decimals, e.g. : 12.3 </td>
        </tr>
        <tr>
            <td><button onclick="changeSitePosition()">Check site</button></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64080,99999)" /> Oostende</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64310,99999)" /> Gent</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64000,99999)" /> Koksijde</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64160,99999)" /> Wevelgem</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64340,99999)" /> Melle</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64470,99999)" /> Uccle</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64500,99999)" /> Antwerpen</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64640,99999)" /> Retie</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64630,99999)" /> Goetsenhoven</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64650,99999)" /> Schaffen</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64770,99999)" /> Diepenbeek</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64320,99999)" /> Chievres</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64780,99999)" /> Bierset</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64790,99999)" /> Kleine Brogel</label></td>
        </tr>
        <tr>
            <td><label for="check"><input name="stations" type="checkbox" id="(64590,99999)" /> Ernage</label></td>
        </tr>
    </table>
  </body>
</html>

working example