Our system automatically tells PayPal that the last page the buyer was on before proceeding to checkout should be the "cancel and return" link URL; this cannot be customized to return the buyer to any other URL.
If you're using the standard E-junkie Cart button codes that display our shopping cart as an overlay "inside" your own page (make sure you paste at least one block of your View Cart code on every page), the "cancel and return" link should take the buyer back to the last page on your site they were at before clicking the checkout button in their cart.