stick your key on the accessory mode, so you get dash lights on.
stick a paper clip in the top 2 terminals (A and B, scope that link and you'll see exactly where to hook the paper clip to)
so, you'll need to bend that paper clip in a U or V shape, and be sure (grab a light perhaps) you are connecting both metal terminals A and B.
after you do that, keep your arm relaxed because it might take a few mins to get them all.
your check engine light will blink morse code. it works by this:
1 short flash = 1
1 long(er) flash = 10
you get the actual number by adding the flashes.
for example:
the first code you get will be 1 long flash and 2 short flashes. this means code 12. each code will repeat itself 3 times and then go to the next (if there is none then it will cycle code 12 all day long).
after a complete cycle of all codes, it will throw you another code 12, 3 times.
to decipher the codes check your Chilton/haynes manual, or scope out:
http://www.gmtuners.com/OBD1_DTC.htm
you'd be looking for a code 36 (MAF error). gl bro.