How does APM and Active directory authentication tie together with the cgnat PPTP

OK. this one is giving me some headaches. I'm setting up PPTP and I know it has to work with APM (we are using active directory authentication). I cannot find how these two things come together. So I have both sides (APM) and PPTP (cgnat, etc..) set up. How do I tie them together? I know that pptp needs a virtual server just like AAA. how do those two tie together?