OK, I see the problem now. 2 things:
1) You are using frames. when you submit the order, it is trying to open the paypal cart page in your framed site, payapl will not allow this.
on the confirm page, add target="_blank" to the form element to force the paypal site to open in a new window:
<form name="checkout_PP_LI_Hid" action="https://www.PayPal.com/cgi-bin/webscr" method="post" target="_blank">
2) your add to cart button is not adding a value to the name cart column, when the item information is passed to the paypal, a name is required.