ok first all of does the engine increase revs when you press the a/c button??? or if u have it running with a/c on and fans blowing you should see the compressor pulley spinning, if not the clutch isnt engaging and ur air con system is not working either a fuse, compressor not plugged in, or a crook compressor, if it is spinning and picking up revs when on, ur a/c system is working but you will need a regass, which just invloves evacuating the system and recharging to the required quantity. and just a caution if compressor is not operating, its possilbe its fucked it self and put metal shaving through out your whole a/c system so it will all have to be removed and flushed with white spirits, and replace tx valve ect, so dont chuck on a new compressor if the old one doesnt go, cause the shavings will only fuck the new one.
cheers