The cart object can be edited using the cart object editor.
The Add to cart button behavior can be edited using the server behaviors panel.
open the page that your add to cart button is on, then open the server behaviors panel and double click the add to cart server behavior in the server behaviors list.
The cart display can be edited using standard Dreamweaver design view.
if you create a new cart column and want to show it's value in the cart display, use the bindings panel to add the cart column binding to the cart display.
Checkout settings can be edited by editing the corresponding server behaviors for your gateway, usually these are on the confirm page for using a remote checkout option.