Hi fishman.

Guess the first question here is....... can you hand fly the aircraft you are flying to a landing?  If not..... then maybe spend some time in the "lessons" part of things and pick up what maybe you have missed there.  To my knowledge none of the default aircraft feature an "autoland" capability.  Also the sim has some great references in the Learning Center for navigation and using the GPS and the ILS.

Typically the autopilot set up correctly and running on "APR" will bring you down the center line and on the glideslope.... but you manually fly the actual flare and landing.  And on the way in, you also typically have to manually configure throttle, flaps, and gear... and watch your airspeed  and VS.

Are you tuning the correct freq for the runway in question on your Nav1 radio?

Also... the autopilot will only engage the glideslope and the localizer if you are "within tolerances" coming into range of them.  Typically this means no more than about a 30 degree angle off the localizer beam horizontally.... and intercepting the GS from "underneath".  If you are outside this kind of setup .... the autopilot can have trouble "hooking up".

Just some thoughts.  Others more familiar with GPS vs instrument approaches will likely have more thoughts.

best,

....................john