You'll need to log in with a Call of Duty account and enter your Call of Duty: Modern Warfare II Early Access Beta code. If you don't have a Call of Duty account, you can create a free one. After creating your free account, you'll be sent an account verification email to the address you provided.
The MW2 beta opens up to all platforms on Saturday, September 24 at 10 am PT / 6 pm BST and ends on Monday, September 26. Starting Saturday, anyone on all available platforms can download and play in the cross-platform multiplayer weekend.