Refrigerant is added to the low pressure side. Assuming your car is still using R-12, there are male screw ports on the rear of your compressor. The one closer to the driver side wheel is the suction (low pressure) side. You can confirm by tracing the adjacent refrigerant hose across the engine and to the firewall. The other (high pressure) hose goes from the compressor forward to the condenser coils in front of the radiator. The best way to charge is by weight, but if you're just topping off, you need to use pressure gauges.

If your car has been converted to 134a, you should have quick connect ports on the back of the compressor with the low and high pressure sides sized differently so they can't be confused.