Make sure to include all the necessary scripts: jquery.js, jquery.validate.js and jquery.metadata.js (if you are using class metadata to define validation rules)? Here's a sample:
<html>
<head>
    <title>Test</title>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
    <script type="text/javascript" src="http://ajax.microsoft.com/ajax/jquery.validate/1.5.5/jquery.validate.min.js"></script>
    <script type="text/javascript" src="http://dev.jquery.com/view/trunk/plugins/validate/lib/jquery.metadata.js"></script>
    <script type="text/javascript">
    $(function() {
        $('#aspnetForm').validate();
        $('a').click(function() {
            alert($('#aspnetForm').validate().form());
     return false;
        });
    });
    </script>
</head>
<body>
<form id="aspnetForm" action="#"> 
    <input name="other" type="radio" /> 
    <input name="details" type="text" class="{required:true}" />
    <input type="submit" value="OK" />
</form>
<a href="#">Validate</a>
</body>
</html>