If you wish a javascript generated popup you define a jQuery-function for this, implement it in
/templates/YOUR_TEMPLATE/javascript/general.js.php (only loading on product_info pages)
and call that function in the cart button using the
onclick-event in
/includes/modules/product_info.php.
e.g.:
$info_smarty->assign('ADD_CART_BUTTON', xtc_image_submit('button_in_cart.gif', IMAGE_BUTTON_IN_CART, 'onclick="YOUR_FUNCTION()"'));
For a PHP fallback you would have to edit
/includes/cart_actions.php and also
/includes/modules/product_info.php.
Regards,
noRiddle