Make sure you have the latest updates to the adobe Extension manager installed.
When you install an extension, open the extension manager, and click the install button, then browse to the ZXP File you downloaded. the error you are reporting will happen if you double click the zxp file to start the isntallation, rather than installing it manully through the extension manager.
also, make sure that you have the deprecated server behaviors extension installed and it is working before installing the WebAssist extensions:
http://www.webassist.com/tutorials/Server-behaviors-for-Dreamweaver-Creative-Cloud
after the deprecated server behaviors are installed and working, install the web assist extensions in the extension manager, when you open DW,, it should launch the WA Installer, if it does not launch automatically, open a PHP page to trigger it.
After the WA Installer completes, make sure to restart DW.
