It's a free download for anyone who pre-ordered the special edition version of the game from GameStop. Anyone else will have to cough up some money, to the tune of $9.99 (or 800 MS Points).

http://www.digitaltrends.com/gaming/dead-island-bloodbath-arena-dlc-is-set-for-release-next-week-on-november-22/

Dead Island ‘Bloodbath Arena' DLC is set for release next week, on November 22