I think it's just to place the targets at the infantry, and then the chopper will fire at it when it sees the target...of course, as bedges already said, you have to use armoured target, since the chopper won't attack infantry targets...
I always used the armoured target (tested all of them) for chopper but i never saw a chopper shoot a shot with his main gun or with his rockets. And with your way, the infantry squad will have to remain at the same position forever when i want them to move.
Here's the invtarget readme (at least the version I got, by Lester, it says).
Too bad, no attachment seem to work for now but you could post the info from the ReadMe file right here. I don't know but i could miss just a init field line and that's why it didn't want to work.
The mission template that i tried was a russian chopper shooting everyone (infantry) on the ground even if they was moving, the script was always adjusting the invisible target position over the head of the squad leader. This script was really great.
Anyway, if anyone who have the instruction or the ReadMe text or a working script, just post it here in quote. It'll help me a lot.
-LeRat