I believe the green wire goes to the O2 sensor's signal wire.
Look at the exhaust header, or near the cat pipe. find the O2 sensor that is screwed into the pipe. there's a long wire attatched to the sensor, and that cable has a harness clip.
I don't know which kind of sensor our cars use yet. there are 1, 2, 3 and 4-wire versions of O2 sensors. if it's a 1 wire, splice into the harness (Not the actual sensor cable, as it's sheilded) on the other types, you splice it into the Signal wire. Look at the service manual of the car to see exactly where to connect this wire.
Anyways, do that, black wire goes to chassis ground, red goes to ignition-on switched positive source, and the last wire (if equipped) go to a switched source for your headlights, to illuminate the gague at night.
Viola! it should work. set the gague according to the manufacturer's directions.
Note, these are not sensitive until fully warmed up. For a better setup, tap a fitting into your manifold, buy a wideband, one wire O2 sensor and hook the sensor up to the gague, won't mess with emissions, and reads your A/F ratio precicely.
Hope this helps.