Just a quick speculation in how you might engage the auto approach! To my understanding in FSX you have certain things to auto approach such as Course selection, NAV/GPS, and of course the approach button. Seeing that Infinite Flight does not have this, do you have a guess on how to activate it. I would presume that it would be, in this order:
Contact ILS frequency (added into the game)
Maintain altitude and speed
Switch to NAV (added into game)
Engage auto approach (added into the game)
Just my speculation that I thought I would share! Let me know what you guys think! Cheers!