EDIT : oops left editor open for too long....
That error occurs when you are trying to get signals from a CAN device you haven't received updates from. If possible you should catch that exception so it doesn't crash your app. Not sure how to do that in Py tho. Maybe someone else can recommend?
Is the PDP device ID the correct value, specified under "PDP CAN ID".
http://wpilib.screenstepslive.com/s/...ribution-panel
Does the PDP show up in the web-based config?
http://wpilib.screenstepslive.com/s/...ribution-panel