Try uninstalling dreamweaver and the extension manager completely using the adobe cleaner tool
http://helpx.adobe.com/creative-suite/kb/cs5-cleaner-tool-installation-problems.html
then installing again.
after you install dreamweaver and the extension manager, make sure to install the Deprecated server behaviors panel:
http://www.webassist.com/tutorials/Server-behaviors-for-Dreamweaver-Creative-Cloud
then open Dreamweaver and ensure that you have the server behavior access points.
once the server behaviors are installed, close Dreamweaver and reinstall the WebAssist extensions.