In a similar move to the other major gaming releases in the past year or so, such as Halo 3 or Call Of Duty 4, Tom Clancy's Endwar is being released as a BETA trial before full release. To get your key you need to submit your email address through their web based form and then wait for them to send you a key. Enter the key in the 'Redeem Code' section on the Xbox 360 marketplace and begin the download. This will take up approximatly 3GB, so make sure you have got the room.

The Private Beta will take place for 3 weeks in June and July, and will be available in English only. The Beta will feature 3 maps and be playable exclusively online in 1v1 and 2v2 matches. The Beta will be available for Xbox 360, and you will need to be an Xbox Live® Gold Member, with a broadband internet connection, and 3 GB of free space on your Xbox 360 Hard Drive.

Be sure to sign up to get your key here. See you on there!
https://www.endwarbeta.com/signup/default.html