1992 Chevy Lumina Location of plugs to install freon

Air Conditioning problem
1992 Chevy Lumina Front Wheel Drive Automatic

My brother has the knowledge and the equipment to add freon to my husband's 92 Chevy Lumina, but they can't find the "plugs" I think they are called, to add it into. Can somebody tell me where they are? THANKS
-- it's getting pretty warm here in Illinois to not have AC!