You must choose them in your loadout, and then if you are on PC it should be bound by default to the number "3" key, and on Xbox and PS3 click the left arrow on the controller's d-pad to use your Stinger or IGLA AA rocket.
The Stinger and IGLA can be affected by perks that the aircraft have such as below radar (if a chopper or plane flies nap of the earth it will prevent or break a missile lock), stealth, and if an aircract uses ECM or IR Flares it will prevent or break a missile lock. The Javelin requires a laser designation from a SOFLAM or tank CITV in order to lock onto aerial vehicles but not for ground vehicles.
