What version of design Extender are you using?
make sure to install the latest version, 1.2.0.
if you have a previous version, make sure to uninstall it from the extension manager.
After installing the extensions, open Dreamweaver, this should start the WebAssist installer. If the WebAssist installer does not start, you may need to open a PHP page to force it to launch.
after the WebAssist installer completes, make sure to restart Dreamweaver.