Hmmm, maybe you don't have the jquery-1.6.1.min.js file - I think I'm going to revert back to using 1.4.1 anyway.
Try changing this line back to the original one
Code:
<script language="Javascript" type="text/javascript" src="clientscript/jquery/jquery-1.6.1.min.js"></script>
To this:
Code:
<script language="Javascript" type="text/javascript" src="countdown/js/jquery-1.4.1.js"></script>
If that doesn't help, PM me a link to your site and I'll have a look.