Problems can occur if say you installed the driver with the PCR keyboard connected. For OS X it's best to just install the driver first and then connect the keyboard. If installed correctly the driver will appear in "Audio MIDI Setup" under the "MIDI Devices" tab even wtihout the USB connection. If you don't see it that would be an indication that the driver didn't install correctly for some reason. What you'll want to do first is remove any residual files from the previous install attemp, they'll be found on the hard drive under Library/Audio/MIDI Drivers -- ex: RDUSB0025Midi.plugin

Remove the folder you find there and then reinstall the PCR driver without the keyboard connected. That should take care of it!