Rigging The System...

[kevin.li.rit]

Well you need code to make the pressure switch work, I.E. You need to program the RC to check and see if the switch is on or off. Then respond by turning on or off the compressor

If you haven't modified the code, then you can try plugging in the compressor spike to relay 8. I'm looking at the default code and it appears to me that they have programmed the pressure switch to work with digital input 18

relay8_fwd = !rc_dig_in18; /* Power pump only if pressure switch is off. */

If that doesn't work, try writing you own code, including the relay you used.
like
if(rc_dig_in18)
{relay8_fwd=0;
}
else
relay8_fwd =1;

Or the same way the default code has it with the relay outputs your using.

Also, if you're using the red and black like Gannon said, then switch it to white and black.