take a piece of wire and use it to jump the connection in the low pressure switch wiring harness/plug. The low pressure swith is on hte passanger side of the car on the low pressure line that you use to service the system (Add R134) hte switch is on top near the firewall, unplug it and jump the harness to complete the circuit.

if this works and your ac is cold, and then you plug the switch back in and it no longer works, replace the low pressure sensor.

this should turn on the AC compressor and allow you to add freon. there is also a couple of wires that go tp the compressor clutch that sometimes break or rub on the fan belt.