If your CAN is hosed (disconnected of wired wrong) you will get a red light on the PCM status indicator. Make sure everything is green.
Check your pressure switch. It should be a normally closed switch, so you should sense continuity (using a meter) across the wires going into the PCM.
If the status is green, and the switch if closed, but are the PCM indicator is flashing showly, and the compressor will not start, then it probably still disabled and you need to look into the code.
Please also remember the code suggestions given above. The compressor WILL NOT start unless either the compressor/PCM is defined in the code or there is some pneumatic element (solenoid) defined.
If you don't have any solenoids defined you will need to define the compressor:
Code:
public status final Compressor myCompressor = new Compressor(0);
We had this problem over the weekend and it took a few hours to figure this out.