I haven't any experience with that radio, but I have got a 3300xlt working without a discriminator tap.
It took a bit of stuffing about with the volumes to get pdw working happily - both radio output and computer input.
pdw seems to like a strong, clear feed from the radio with no distortion from high volume, and radio location to ensure a clear signal also impacted my setup.
I think I got it all plugged in, then spent some time working through the full volume range of the radio. When garbled text started appearing on the screen, I could fine tune the volume adjustment.
For memory there are test pages sent every minute or 30secs, so you have a fairly constant stream to let you tune the system.