The thread title is incorrect. Halo is in the Games app, but not in the Store yet (in most countries). It was added to the Games app about a week ago, probably coinciding with when the game was available in Trinidad & Tobago. It is not published in the US Store yet.
when ever i go to that page and press the play button it comes up with a message saying i dont have the right app!!!
but doesn't tell me which one i need.
Can you help?
The app you need is the game itself. When you tap on the button to look for it in the store, it searches for it, but doesn't find it, because it's not there yet. Normally, once the game is released, tapping on that button would automatically bring up the page to buy it.