Get the Jaguar Getting Started Guide here
http://www.usfirst.org/roboticsprogr...control-system if you haven't already. Check the Jaguar LED status indicators (page 7 in the Getting Started Guide) for a better indication of the problem.
The most likely problems:
It's not clear if you're not getting communications or if it's just that there is no motor output happening. If it's just no output (but the lights go solid when the robot is enabled and blink green or red when commanded to drive a motor) then check your limit switch jumpers -- they MUST be installed or there will be no output voltage to the motors.
If your limit switch jumpers are installed correctly, check your CAN cable pinout; you can not use standard 4-pin telephone cables. verify the pinout in Appendix A, pages 23-25 of the Getting Started Guide and verify that your cables are made correctly. Also, looking at the front of the Jaguar, signal comes in from the previous Jag via the left-most connector and out to the next one via the right-most connector.