If you're me, and you've tried to install the DayZ mod into Arma II, then you made a real hash of it. Fortunately for all the me out there, there's a simpler way. The mod has been made available directly over Steam, meaning you can just click a couple of buttons and all the scary work is done for you.
You'll need to own both Arma II and Arma II: Operation Arrowhead, or indeed the bundled version, Arma II: Combined Operations, to make it work. If those are installed, and have been run at least once before, then getting the DayZ mod going is so simple that even I just did it. No, I really did. I clicked to download the mod, then clicked Play. And there it was.