To access the Concept2 PM3/4/5 over a usb connection you also need to install the PyUSB library onto your Raspberry Pi. See below for the most recent version tested:

Python.org - pypi.python.org/pypi/pyusb/1.0.2*

Old PyUSB archive: 

Walac PyUSB - https://github.com/walac/pyusb

*Thanks to Mark Pettit for this notification