this error usually happens if you install one of the stand alone extensions after installing Design Extender or Data bridge.
Design Extender or Data Bridge should be the last extension you install.
see this thread for install tips.
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.