mando_airsupport_cmissile and mando_airsupport_cmissile_pos are not files, are global variables as explained in the online help.
You may set them to the values required by your mission in the init.sqf file (never inside description.ext as wrongly suggested by Luke).
You need a launcher unit (the cruise missile launcher) in your map. Each side may have only one missile launcher.
The missile launcher is assigned to mando_airsupport_cmissile and the firing position is assigned to mando_airsupport_cmissile_pos global variable, just as in the example (part of init.sqs):
mando_airsupport_cmissile = rhib1; // <- This is a ship NW of the island.
mando_airsupport_cmissile_pos = [0,-4, 1]; // Firing position (where the missile will be created at launch time)
mando_airsupport_cmissile_alt = 300; // Cruise altitude of the missile to keep in-flight
rhib1 is the name given to a ship that will work as missile launcher in that demo mission.
[0,-4,1] is the firing position in model coordinates [just 4m behind the center and 1m above the center of the firing unit).