Instructions
Attain a killstreak. A message will pop up on your screen when you have obtained a killstreak reward. Stay out of open areas if you want to survive long enough to get a streak.
Press "Left" on the directional pad for the Wii remote. The directional pad is the cross-shaped button over the "A" and "B" buttons near the top of the remote.
Move the analog stick on the Wii nunchuck to the right to activate your killstreak reward. Sometimes you will not have to do anything (like with a harrier reward), though other times you will have to toss a smoke grenade to mark a location (like with a supply drop) or control a predator missile.