not really enough info to work with. however. if you really have 0 pressure. then your compressor will not turn on at all.. nothing came out of?? the can? or the system? anyways.. make sure your actually opening the valve on the system.. some couplings don't seem to reach far enough to open to valve. you migth even go so far as to open the valve without the kit./. ( keep your self clear of it. eyes faces especially.. ) most often it's very similiar to a tire valve. if it sprays out.. then you know for sure you ahve pressure. and that your kit is not connecting correctly too.
if all else fails i'm sure the kit came with directions.. re read them.. just to be sure of how it all works. an try it again. hoepfully you'll at least get a reading for your pressure.. usually there is "some" pressure in the system.. even when they have leaks. best of luck!