Make sure that you have the server behaviors extension installed:
also, make sure you have the latest versions of the web assist extensions installed:
Design Extender 1.1.7
Data Bridge 1.1.8
if you have previous versions installed, you should uninstall them before installing the newer ones.
after the install competes, open DW and open a PHP to start the WebAssist installer.
after the WA Installer completes, don't forget to restart DW.