After installing the extension in the extension manager, the WebAssist installer needs to run to complete the installation before you can use it. It sounds like the WebAssist installer has not run yet.
Close DW, and open the Extension Manager.
In the Extension Manager, uninstall Free Tools, then install it again.
Open DW, the WebAssist installer should launch, if not, open a php page to force it to launch. After it completes the installation, restart DW.