In order to be able to play Day Z you need to have legit version of ArmA II and the Operation Arrowhead add-on (also known as the Combined Operations pack). You can use either retail (disc-based) or Steam version of both games.
Official mod website is available here for you to check. Feel encouraged to do that at least once a day since currently mod updates are being released almost every day. Be sure to have the latest mod version installed before you enter the game.
Installation instructions for Steam edition owners:
- Go to http://dayzmod.com/
downloads.php, choose a mirror and download the 7 .rar files that make the mod.
- UnRAR them somewhere. You should get 14 new files – 7 .bisign ones and another 7 .pbo ones.
- Go to your Arma OA folder inside the Steam folder and create a new sub-folder called “@dayz“.
- Inside the “@dayz” folder create a new folder called “addons“
- Put the 14 dayz files, you unpacked earlier, inside that “addons” folder.
- Then go to your Library in Steam client, right-click Arma II OA and select Properties.
- Select SET LAUNCH OPTIONS and enter -mod=@dayz -nosplash -noworld
Now you should be ready typ play. Enter the game and try joining a MP session of DayZ (use Filters to find DayZ servers more easily).