reset the workspace layout:
go to Window -> Work Space layout -> Reset '<your layout name>'
and Delete the Dreamweaver Cache file from the users configuration folder:
http://www.webassist.com/tutorials/Common-installation-issues
NOTE: Before installing Design Extender, you should make sure any of the following extensions are uninstalled:
-- CSS Sculptor
-- CSS Menu Writer
-- Framework Builder
-- PowerCMS Builder
-- Pro Maps for Google
-- Site Import
-- Site Sculptor
-- Social Media Toolkit