Black Ops killstreaks!!! How to use it?

hi guys; Black Ops is my first COD game in like 5 years (level 35). I'm having trouble with the killstreaks. I bought the Sam turrets/sentry gun but i don't know how to get them; I heard the heli drops it but every time i wanna mark a place to drop the Sam turret nothing happens and it's like I have just a Stun grenade :s Do you have to mark a specific location?? (sorry for the dumb question but i'm kinda new to cod on consoles)