Launch Arma 2: Operation Arrowhead from the Steam library, and run it as Arma 2: Combined Operation.Launch Arma 2 (you need to run Arma 2 first in order for Arma 2: Combined Operations to work, which is required for DayZ.Make sure they are in the top level of the Addons folder or Arma 2 won't be able to find the files. Create a folder inside of that folder called Addons, and then download all the DayZ mod files from the DayZ website into that folder (So they should be located in C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation Extract the files into the Addons folder.
You'll get a bad serial key error otherwise) I just got this working myself, although it took some help from the internet to get it working.
