Make sure that you have all updates to Dreamweaver and the extension manager installed.
Then 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:
after deleting the cache file, uninstall Design Extender for the extension manager and install it again.
When you open Dreamweaver it should launch the WebAssist installer. if the WebAssist installer does not open automatically, open a php page to force it to run. after the webAssist installer completes, make sure to restart DW.