On this page they reference supplying it with 24 volts:
http://wpilib.screenstepslive.com/s/...dware-overview
Oddly, despite having FIRST choice options for 24V solenoids as well as having them in the rookie kit, they do not have a diagram for how to wire for 24V solenoids.
Here is the diagram from 2012:
I would suggest submitting a Q&A question about it to confirm that this is the proper procedure.