First off, I have to tell you.. the little gauge that comes with the recharge kits are very ,very innacurate. They are more of a marketing ploy than an actualy functional tool and really tell you nothing more than if there is refrigerant in the system. In order to properly diagnose an A/C system, you need a set of professional gauges that will read the pressure on both the high and low pressures sides of the system.
With that being said, I highly recommend having the system professionally evacuated and pulled down to full vacuum before servicing, but if you choose not to do so I recommeng installing a temperature gauge in your center vent, and with the A/C on the full cold and high blower setting, add about 1/4 can at a time until you notice the temperature no longer continues to decrease. If you overcharge the system, it will not operate correctly and can damage A/C components.
The A/C lines coming off of the compressor contain the service ports. There are two.. a high pressure (discharge) port, and a low pressure (suction) port. You MUST install the charge hose to the suction port. Luckily, with R134a refrigerant, the ports are of different sizes, which makes it almost impossible to install it on the wrong one.
It appears the low pressure (suction) port is at the rear of the compressor. This is where you will add the refrigerant.
The high pressure (discharge) port attaches to the front of the compressor.. do NOT attempt to charge the system here.
Refer to the following illustrations. Good luck!