Hey,
Assuming you're using this PDB:
http://www.usfirst.org/sites/default...tion_Board.pdf
I don't actually see anywhere to hook up a CAN line. The PDB I linked doesn't have current monitoring or anything that would require data being sent back and forth from the roboRIO to the PDB.
That said, the exact mechanism to attach a 120 ohm resistor to the CAN circuit will vary. If you're using a talonSRX, for example, you could simply strip some wire back from each of the lines (green and yellow), and then solder a resistor onto there, with some heat shrink applied around there to prevent shorting.
As for the 120 ohm resistor, I believe that's a requirement. AFAIK it was fairly arbitrarily chosen, but since then it was standardized. The resistor is there to dampen noise, I think. Larger, and the signal itself will be attenuated. Smaller, and the noise will be exaggerated.
Above corrected by the helpful posts below.