Try this (I just tried it) also with C172.
On your Map, select "IFR low enroute".
Zoom in on JFK and find your runway 31L and its IFR zone.
Click on that runway's yellow triangle. You'll get a box for "KJFK 31L ILS-cat-I (IMOH)", click "Tune NAV1".
Also, on the Map, click in "Approach" and select your runway "KJFK 31L ILS-cat-1".
To see your ILS localizer beacon, hit CTRL-P (path) 3 times. The 3rd time will light up your approach box.
It's easy to see those boxes, and ride them down to the runway.
Finally on your autopilot, you hit "APR" and "AP" and it will fly for you. Cut your throttle to control your glide speed.
After all that, here's the catch -- --
On my X-Plane,
-- the visual boxes always work, if the runway has ILS.
-- the autopilot's ILS works on some runways, and not others.
Maybe one of us should submit a bug report. I have not done so.
XP view of JFK 31L:
Google Earth view of JFK 31L: