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:
Then uninstall Design Extender from the extension manager and install it again.
when you open DW, it should launch the WebAssist Installer, if the WA Installer does not launch automatically, open a PHP page to force it to run. when the WA Installer completes, make sure to restart DW.