do you have any of these extensions installed:
*CSS Sculptor
*CSS Menu Writer
*Framework Builder
*PowerCMS Builder
*Pro Maps for Google
*Site Import
*Site Sculptor
*Social Media Toolkit
if so, you will need to uninstall them and design extender, then Delete the Dreamweaver Cache file from the users configuration folder:
common_installation_issues.pdf
after deleting the cache file, reinstall design extender.