Make sure that you have the latest version of Design Extender installed. If you have a previous version installed, uninstall it from the extension manager before installing the newer one.
also, 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/community/tutorials/view_tutorial.php?tid=95