Have you intialised the collapsible panel?
Looking at your source you do not appear to have initialised the collapsible panel
You need to add this after the collapsible panel </div>
<script type="text/javascript">
var CollapsiblePanel1 = new Spry.Widget.CollapsiblePanel("CollapsiblePanel1");
</script>
For more information see point 8 at collapse_panel/
regards
Jon