You need to remove the initial form from the page before adding the add to cart buttons.
See the following tutorial:
http://www.webassist.com/tutorials/Add-a-shopping-cart-eCommerce-Series
in the Section titeld "Delete existing Add to Cart Button"
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.