Yes, those a/c recharge kit works pretty easy and simple, however, ONLY IF you fully understand what you are doing.

Do keep in mind that running low on freon do not cause too much problems, just the pump works harder. However if you over charge the freon, you may blow up the ac system, I'm talking about explosion here.

Most of the recharge kit only comes with one gauge, which tells you the pressure on the low side only, and the pressure changes a lot as the a/c pump goes on and off. You have no idea what is the pressure on the high side, and it won't tell you how much freon you already have in the system. As marauderroy said, you should have 2 gauges on both low and high side to recharge it safely.