The HeloHunter
 

I created test mission with US Sub (SIDE 0) and Heliport at land with 1 helo inflight (SIDE 1). Then I made triger attached to helo, type GOAL, KILL, by side ANY with short text report "Helo destroyed" for ownship. Then I started this mission.
After helo come to STINGER range, I destroyed them, but the trigger has not been fired and this goal don't appear in the debrief screen. Where may be error ?

I think there's an 'apply to' section that might screw it up, if it's been applied to something other than the sub. It sounds like you did it right, so other than maybe not saving it after you put the trigger in, that's all i can think of...