It looks like you're trying to use the PBDOM extension as well. You need to make sure you have that PBD (PBDOM105.PBD) in your library search path or import the PBX (PBDOM105.PBX) into one of your PBLs.
Is the service really returning a string that you need to parse with PBDOM? The web service client is intended to retrieve the response into a structure that matches that of the web service response if there is a complex response.