Jquery not working

Are you sure the jQuery library is loaded on your page? The error you’re getting implies it isn’t. View the source and search for jQuery.

Then try adding this some where in your functions.php (or plugin) file and see if your script works.

<?php
add_action( 'wp_enqueue_scripts', 'wpse30127_enqueue' );
function wpse30127_enqueue()
{
    wp_enqueue_script( 'jquery' );
}