I'm sure you've seen the API docs already:
http://robotpy.readthedocs.org/en/la.../CANTalon.html
My understanding is that you have to switch the talon into the correct control mode, set the correct PID parameters, and then you just can just call "set" with the desired control parameter (in this case, velocity).
I feel like I saw somewhere in the forums mention about a firmware update for the talons, so make sure those are up to date also.