after adding a new column to the cart object, you need to edit any page that has an add to cart behavior on it. On the server behaviors panel, double click the add to cart behavior and set the value for the new cart column.
if you continue to have a problem, send a link where i can see the issue, a copy of the /WA_eCart/pop_cart.asp file and a copy of the page with an add to cart button on it.