The first expansion pack for XCOM: Enemy Unknown lands on PlayStation Network, Xbox Live, and PC December 4th. It will be available for $6.99 (560 Microsoft Points).
As previously announced, "Slingshot" adds three new missions to the campaign set in China, promising to add a bit more structure and narrative to the mostly randomly-generated game. To access the add-on's new content, you'll need to start a brand new game; the first opportunity to partake in a DLC mission should occur around when you make your first encounter with a downed ship.
The remaining DLC missions will appear as optional objectives in your command center as the game progresses. Successfully completing the missions will unlock a new squad member for your team and earlier access to certain bonuses, therefore changing the way you approach your upgrade tree for the entire game.
More DLC for Enemy Unknown is planned for 2013.