With both downloaded and installed, you'll need the mod files.
The manual method: mod ArmA 2: Operation Arrowhead For more advanced instructions, check out the official guide to using Six Updater and the tool's documentation. It's a piece of software that automatically manages your ARMA II mods - once you've got it up and running, installing DayZ is as simple as choosing a server and waiting for the updater to pull down the necessary files. Whichever method you choose, you'll need to load Combined Operations at least once while running Steam as Administrator. It's easy to find: the mod's so popular that it's launched Bohemia's mil-sim into the Steam top ten. First up you need to own ARMA II: Combined Operations, that's both ARMA II and Operation Arrowhead.
