Customizing Cart Page

Hi,

I am trying to customize the Shopping Cart page on Marketpress by making a mp_cart.php page and using mp_show_cart('page') to display the innards of the shopping cart. Everything displays as expected, but I cannot actually check out from this page -- when I click the "checkout" button, the page refreshes but nothing happens (likewise, the "empty cart" button does not work, albeit in a different way).

What am I doing wrong?