Very seldom can you fix an AC system by shooting in a $15 can from the local auto parts store. It might help for a while, but that's just a temporary fix.
Like Stangman said, If the compressor is not engaging jump the low pressure switch connector for a brief moment and see if the clutch works. If it doesn't, check for voltage at the clutch connector. If you have voltage the clutch is bad. If you don't, check the AC fuse and go back a couple of steps. If the clutch does click on with the switch jumped out then it's either the switch itself or a low charge. You'll need some guages to determine the charge/blockage in your system. If it's completely void of freon then you'll have to repair the leak, replace the dryer, completely evacuate the system, leak test it, and charge it. Personally I would replace the orfice tube and flush the system while it's open.
Of course you could always go to Autozone and get a can of freon with leak seal/dye and screw the system up worse.
Just my opinion after working on several AC systems new and old.