After installing the extensions to the extension manager, make sure that the WebAssist installer opens to finish the installation.
you may need to open a php page to open the WA installer if it does not launch automatically.
The error on start up is probably because a WA floater was enabled in the workspace, and would probably be fixed by letting the WA installer complete the installation.
It looks like you tried using the server behaivors before the WebAssist installer completed.
Uninstall the extensions from the extension manager, then reinstall them.
The install Procedure in DW CC is this:
1) make sure the Deprecated server behaviors extension is installed. if not install them:
then start and close DW
2) install the Web Assist Extensions
3) Open DW, this should launch the WebAssist installer. if not, open a PHP page to force it to launch.
4) after the WebAssist installer completes, restart DW.