The markercluster takes an array of google.maps.Marker objects, the "all" array is an array of numbers onto the end of which you push some google.maps.Markers.
function initialize() {
var pos = new google.maps.LatLng(-39.431441,-71.286622);
var myOptions = {
zoom: 4,
center: pos,
mapTypeId: google.maps.MapTypeId.ROADMAP
};
var map = new google.maps.Map(document.getElementById('map'), myOptions);
map.setCenter(pos);
var all = [
[-41.8675, -73.827697],
[-23.65, -70.4],
[-42.48014, -73.762414],
[-20.2167, -70.142223],
[-29.906301, -71.250204],
[-39.272254, -71.977629],
[-41.3167, -72.9833],
[-22.908707, -68.199716],
[-33.4625266, -70.6512506],
[-39.819586, -73.24521],
[-33.045646, -71.620361],
[-33.024527, -71.55234],
[-43.1216233, -74.0300122]
];
var gmarkers = [];
for (var i = 0; i < all.length; i++) {
var lat = all[i][0];
var lng = all[i][1];
var latLng = new google.maps.LatLng(lat, lng);
var marker = new google.maps.Marker({map: map, position: latLng,});
gmarkers.push(marker);
}
var markerCluster = new MarkerClusterer(map, gmarkers);
};
google.maps.event.addDomListener(window, 'load', initialize);