Don't give up RodTod. You may find that your $50 bucks were totally worth it.
I don't have the C172 but I do have the C182, with the same Autopilot.
A2A aircraft are the best there is in GA for FSX and Avionics is so well modelled that you can use the real Manuals from the real Manufacturer.
In this case, get the KAP 140 Autopilot System Pilot's Manual from Bendix King.
You can also download it from the A2A Forum where you will find lots of info about avionics, including tutorial videos.
Cheers,
PS. Just keep in mind that a 2-axis Autopilot doesn't fly the aircraft for you (doesn't have control on throttle, has a limited bank authority, etc) so, as a rule, you should have your aircraft manually set for a certain flight condition and use the Autopilot just to give you some (just some) freedom to take care of other things, or enjoy the view.