Try reinstalling the webassist extension you have installed. That file should be added with any of our extensions, so if it is missing that means the installation didn't work properly.
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.