Wher do I find the Low side fill port for air conditoning system?

If you can't find it, you shouldn't be looking for it. Handling refrigerants is serious business. Professionally, a certificate of certification per Environmental Protection Agency section 609 is required before a technician in a repair shop can even work with air conditioning systems. Please seek the assistance of a local, qualified technician certified to handle refrigerants and diagnose air conditioning systems. Have you done a diagnostic test of the system to see if it is even low in refrigerant? Have you vacuum tested the system for leaks? Adding refrigerant to a system where leaks are known to be present is environmentally irresponsible and a waste of money.