First off, there are 2 lines. One low pressure; one high. If you connect to the wrong one, the can might explode in your hand or face.
Secondly, a lot of folks assume that because an a/c system isn't cooling that it's low on refrigerant. That may be the case, but not always.
The only way to determine what's going on is to hook up a set of gauges designed for the refrigerant in use. R-12 and R-134a gauges are different.
After the gauges are hooked up, you must interpret the high/low side readings. These readings will tell the story.
One needs training in order to do this correctly.
It would be best to pay a shop to hook up some gauges and tell you what they see.
If you hook up one of these "kits" and low refrigerant is not the problem, you'll over- charge the system. There is a high pressure switch in the circuit that detects excess charge. When this happen, your system "short cycles" or turns on and off rapdily and cooling never takes place.
Again, just because it's not cooling doesn't mean it's low on juice.