you have to initiate the plugin like this
jQuery(document).ready(function($){
$("abbr.timeago").timeago()
});
the final code will be like this
<html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="jquery.timeago.js"></script>
</head>
<body>
<abbr class="timeago" title="19 March 2013 04:51:33 PM"></abbr>
<script>
jQuery(document).ready(function($){
$("abbr.timeago").timeago()
});
</script>
</body>
</html>
hope this helps