There is a hidden form element on line 294:
eCart5_1_Quantity_Add
it's value is blank. Set it to "1" and the add to cart should start working.
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.