did you have previous versions of the stand alone extensions installed?
design extender includes these extensions:
-- CSS Sculptor
-- CSS Menu Writer
-- Framework Builder
-- PowerCMS Builder
-- Pro Maps for Google
-- Site Import
-- Site Sculptor
-- Social Media Toolkit
if any of those are installed, they need to be uninstalled before you install design extender.
also, Delete the Dreamweaver Cache file from the users configuration folder: