The error suggests the WebAssist installer has not run to finish the installation.
recreate the users configuration folder as outlined in step 12 of the Adobe troubleshooting guide:
After recreating the configuration folder, open Dreamweaver, go to Commands -> Manage Extensions, then close DW.
In the Extension manager, select each extension and remove it.
Then click the install button to browse to each extension MXP File and install them.
After installing the extensions, open Dreamweaver, this should start the WebAssist installer. If the WebAssist installer does not start, you may need to open a PHP page to force it to launch.
after the WebAssist installer completes, make sure to restart Dreamweaver.