There should be a terminating resistor at each end of the CAN bus. The roboRIO has one built in (see page 9 of
the roboRIO user's guide), and most 2015 FRC robots used the one built into the PDB. Using an older PDP, you will have to add your own terminator. This is simply a 120Ω resistor is connected to the green wire at one end and the yellow at the other. As this is just provided to make the impedance correct for how the lines are driven, it shouldn't be crucial that it be exactly 120Ω, but I wouldn't just use a 100Ω or 150Ω resistor either. There would certainly be no problem if you put (for example) a 100Ω and a 22Ω resistor in series with each other to make 122Ω.
Physically, I would recommend a few inches of green wire, the resistor(s), and a few inches of yellow wire, then plug this into the empty connectors of the last device on your chain.
Added: In case it wasn't clear, the green and yellow wire aren't important electrically, but they help communicate the purpose of the resistor to the controls team.