It sounds like the WebAssist installer has not run to complete the installation.
Close DW< and open the extension manager.
in the extension manager, uninstall Design Extender then install it again.
After installing the extension, 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.