Find your AC compressor. You'll see two lines - one will be larger, and one will be smaller. The larger one is the low pressure line going into the compressor inlet. You just trace that line back until you find the schrader valve (if your AC is a fixed orifice tube system, it's likely to be on the accumulator).
It is located near the right shock tower - the cap should be marked with an "L".
Under the hood, right side, in low pressure line.
The Villager does not have a test port. You need to tee into the line at the fuel filter.
Under the dash to the right of the steering column.
The Villager does not have a test port, you need to tee into the line near the fuel filter to test pressure.
Through the low side A/C port.
Under the dash to the right of the steering column.
directly below the steering wheel at the bottom of the dash
Error codes on a 1997 Mercury Villager can be found by connecting an OBDII code reader to the diagnostic port. The service manual has a full list of error codes and their causes.
The OBDII diagnostic port is under the dash to the right of the steering column.
You must have a code reader to interface with the OBDII data port under the dash.
Freon is added to the lower pressure line port. It is the only port a refill kit will fit. I suggest taking it to a shop that does A/C repair if you do not know that already.