Well first make sure your headset is working, I do this by creating a message and adding voice to it because that way you can make sure the microphone is working.
If it is working then it should just be a simple matter of speaking while in game and you should be heard.
seems to be a problem with game code or personal setups, Played many co op games and certain people cant hear one another while others can, I was having to pass messages on to people i could hear but others could not
I also have this problem. Very frustrating. Makes you wonder how this game got certified? The amount of problems and the rate of reoccuring problems is someone's kind of a sick joke!