What is your OS?
What version of Dreamweaver?
What version of Design Extender is installed?
make sure you have the latest version, 1.1.7 installed. If you have a previous version, uninstall it from the extension manager and install the latest version.
Note, the new version uses a new install routine.
after installing the extension in the extension manager, open DW< then open a php page to launch the WebAssist Installer that will finish the install process. After the WebAssist Installer completes, restart Dreamweaver.