install the latest updates to Data Bridge, eCart and Design Extender, the latest versions are:
Design Extender 1.2.0
Data Bridge 1.2.1
uninstall the previous version from the extension manager, the download and install the newer versions. 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.