A/C not blowing cold air, how to add the refrigerant?

If you have zero pressure you have a leak, and it would be a waste of money to try to recharge it until the leak is repaired. You need to find the leak and repair that first, then pull a vacuum on the system and be sure it runs long enough to pull all the moisture out of the system before you try to charge it. You know it's a leak and not just low because of the pressure. If there is even a small amount of 134A in the system, it would show a pressure equal to the temperature of the air around the vehicle. So, if it is say 89 degrees outside, you would see about 87-90 psi when the gauge is connected. In this case, 0 means it has a leak, and a leak in an AC system can work both ways.
Finding a leak can be difficult but in many cases, you can look for an oily area where the oil leaked out with the 134a. Connectors are common, as are the valves. Next are the cores. If you have an oil free compressor and a couple water traps on the line you can also add about 100 psi of air to the system, then spray everything down with soapy water and look for the bubbles. Then repair and vacuum it down for the new charge.

Leaks are unfortunately a common thing in A/C work. Multiple connections with seals, materials that corrode and compressors seals and even compressor bodies all can be sources of leaks. It can make you want to take up drinking, trying to hunt them down sometimes. Then with the quality of parts these days I've installed brand new parts that leaked worse than ones I was taking off!