Where is the low pressure port on my Lincoln MKZ?

Do not try to recharge your 2008 system. It should not be low yet and overcharging will damage the compressor. In fact, it is far easier to damage the compressor than to properly charge the system at home. If it is indeed low it has a honking leak and that will have to be repaired first anyway. If the system is not working right it is a job for a professional.