Everything is handled through an action menu added to the desired units (planes, choppers). So you only need to make sure that action is available for every player connected.
The easiest way to do that is just to add the action in the init field of each ILS capable vehicle (just like the one present in the demo mission):
this addAction ["Call ILS", "ofpec_ils\ofpec_open_airfieldselector.sqf"]
But if you are using dynamically spawned units, then you need to add the action in a different way:
// Your spawn unit code hereThis has the same effect of adding code to the init field of a unit placed from the editor on the map.
_spawnedunit SetVehicleInit "this addAction [""Call ILS"", ""ofpec_ils\ofpec_open_airfieldselector.sqf""]";
Note that you might have, for example, pilot and co-pilot of a Su34 both with the ILS action enabled, so even the gunner might activate it and follow the final landing aproach (while he cannot drive the plane).
If needed, you may add the action only when a pilot-soldier type player is in th driver seat of a plane or chopper, all that is up to the mission designer.