what is the pressure switch?

how is wired

You take a PWM cable and use two of the wires to wire to the switch. Everybody uses digital in/out 18.

The pressure switch (assuming it hasnt changed is a black cylender about an inch tall and an inch in diameter(give or take) on one side it has a brass threaded fitting (just like all the other pnuematic stuff), and on the other end it has 2 brass electrical leads.

caraddicted is right, If I recal it should be the white and black wires that you use. The only reason people use digital input 18 is because it is set up to turn off the last relay (I forget the number) in the default code. Basically, if you plug the switch into dig in 18 and the compressor spike into the last relay you dont need to change any code.