This is an issue that I have heard about but not experienced myself. The fix should be to delete the configuration folder, then remove the extension left overs from the extension manager. Deleting the configuration folder should also delete the menu file that is giving you the error.
After you do this you should try deleting the cache file and then restarting your machine. This should leave you with a Dreamweaver that has no extensions in it and allow you to install more extensions. When you install them you should start with Security Assist.