i forget the specifics with the switch, and also am weary of telling novice people to attempt this, but..

the pressure switch is on the silver line right near the intake, you will see the plug. now to test if your ac is a pressure problem or not, you either unplug the switch, or connect the pins with a paper clip. then try turning it on, see if it is cold.

do not let it run long, under charged ac will eventually damage the compressor.

if you do this, and the AC kick on and runs, and get cool air out of the AC, then you either need a charge, or need a pressure switch. if the system kicks on, but you do not get cool air, you most likely have a leak in the system and no r134 left. if the system does not work even after tripping the switch, then you have another component issue.