Yeah, the only mode with AA missiles is the Longitudinal aiming mode (6 i think), the Su 25 doesnt have any RADAR, so there's no trully AA mode ... you have to lock with the missile sensor before attempt a launch.
Hint: In Longitudinal Mode, the cross juste below the center of the reticle IS where the head of your missile is pointing to, try to put your target on that cross, when you hear the tone, FIRE! :)
Although just remind that the Su25 is not fitted at all for the AA combat :)