A/C recharge

I suspect that the A/C system on my '99 Sable needs a recharge. There's no cold air and the compressor clutch doesn't engage. Fuse is good. I'm not sure where to connect to the low pressure side of the system. There's a port with a green cap near the fire wall that says "evap service port", and a port with a black cap nearer to the compressor. Anyone know which is which? Thanks

NOT the evap port. On my 97 it is the port near firewall on pass side.