before installing the WebAssist extensions, make sure to install the depricated server behaviors extension:
for DW CC 2014:
http://www.webassist.com/tutorials/Deprecated-Server-Behaviors-for-Dreamweaver-Creative-Cloud
For DW CC:
http://www.webassist.com/tutorials/Server-behaviors-for-Dreamweaver-Creative-Cloud
after installing the deprecated server behaviors, open DW and close it again before installing the WebAssist extensions.
When you complete installing the WebAssist extensions in the extension manager, open DW to trigger the WebAssist installer, if the WebAssist installer does not launch automatically, open a php page to trigger it.
once the WebAssist installer completes, make sure to restart Dreamweaver