looking at the source of your page i don't see the WA_Store_Cart_1_ID_Add hidden element.
looks like this code was removed from the webassist/plugins/shopping_cart/add_to_cart.php page.
<input type="hidden" name="WA_Store_Cart_1_ID_Add" id="WA_Store_Cart_1_ID_Add" value="<?php echo $row_WADAProducts["ProductID"]; ?>" />
add it back just before the submit button