how to engage ac clutch when not engaging so we can add refridgerant

IF the proper system charging ‘procedures’ are being followed and the compressor is not engaging - then there must be a malfunction ‘elsewhere’ that is preventing compressor engagement .. Pressures contained within the refrigerant canister will be plenty sufficient to force contents into system and the compressor to engage.. That’s just how it works - on an otherwise functional system. Good luck!