You can use one of those, but there are a lot of different reasons why it might not be blowing cold. Is the condenser in good shape and has plenty of airflow? Is there a restriction in the system? (feel the tubing for the high side, it shouldn't get cold until the evaporator, or right before. If it does, or there is frost on the lines it indicates a restriction usually). Are the blend doors working properly? The proper way to do it is to evacuate the system and pull a good vacuum on it and see if it holds for 20 min. If it does then there aren't any leaks, at least any big ones. Then charge it with the proper amount of refrigerant.
But since I'm fairly certain you don't have a $4k+ AC service machine, the next best thing is to use a manifold gauge set, you can get them for like $50-60 at HF, good enough for home use. Hook it up properly with it on Max AC and watch the high side/low side pressures as the compressor cycles on and off, generally if they are a little low then you are low on refrigerant.
Or just get the parts store kit, and hope for the best. Overcharged is just as bad as undercharged though, and sometimes worse.
