Replicating Gridmarket's Sliding Cart Header

Hello,

I'm trying to replicate the sliding panel that reveals the shopping cart when "view cart" is clicked in the header of the gridmarket theme.

I've got everything working just fine except for one minor detail. Where it displays "cart: 0 item(s)", it remains displaying zero until you click to another page or the current page is refreshed. products which are added to the cart appear instantly and are added perfectly, it's just the jquery/ajax auto-update for the number of items in the cart that isn't functioning.

You can see it in action here:
http://bit.ly/qGxMep

Here is my header.php code

<!-- BASKET HEADER -->
<div id="top" class="header-wrapper">
  <div id="header-basket">
    <?php if ( class_exists( 'MarketPress' ) ) {
						?>
    <div id="cart-contents">
      <?php _e( 'Cart:', TEMPLATE_DOMAIN ) ?>
      <span><?php echo sprintf(__('%s item(s)', TEMPLATE_DOMAIN ), mp_items_count_in_cart()); ?></span></div>
    <div id="mp-cartsmall">
      <div id="toggle"> <a id="open" class="open button" href="#">
        <?php _e( 'View Cart', TEMPLATE_DOMAIN ) ?>
        </a> <a id="close" style="display: none;" class="close button" href="#">
        <?php _e( 'Hide Cart', TEMPLATE_DOMAIN ) ?>
        </a> </div>
    </div>
    <?php } ?>
    <div id="search-bar">
      <?php get_search_form(); ?>
    </div>
    <div class="clear"></div>
  </div>
</div>
<?php if ( class_exists( 'MarketPress' ) ) {
					// sliding shopping cart
					?>
<div id="panel">
  <div id="panel-inner" class="mp_cart_widget"> <?php echo mp_show_cart('widget'); ?>
    <div class="clear"></div>
  </div>
  <div class="clear"></div>
</div>
<?php } ?>
<!-- /BASKET HEADER -->

Many thanks