The add to cart button is just a place holder.
To add eCommerce functionaly, you will need eCart. see this tutorial for details on using the 2 extensions together:
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.