You will need to have your form post to the page with the add to cart server behavior on it. In the add to cart server behavior in the bindings tab you need to associate each column in the cart with it's corresponding form element using the lightning bolt icons.
If the form is on another page you may need to go to bindings > Form Data on the page that form posts to and select the form page. This will put the bindings from that form into the bindings panel on this new page. You can then associate the columns of the cart with the new forms elements from the bindings panel.
