wordpress + jquery syntax error

Solution:1

WordPress’s packaged jQuery defaults to noConflict mode, use jQuery('document'), or wrap your code like

(function($) { 
    // Code that uses jQuery's $ can follow here.
})(jQuery);

or in your document ready like this:

jQuery(document).ready(function($) {
    // Code that uses jQuery's $ can follow here.
});

Solution:2

try jQuery instead of $

jQuery('document').ready(function($){
    alert($('.thickbox'));
});