Dudes at the front page are yankin your chain.
You don't need Combined Operations to play Dayz, all you need is the Demo & the first Expansion Pack (Operation Arrowhead) You can download the Demo off there site, then buy either the retail version of Operation Arrowhead or the Steam version.
The tutorial on making it work perfectly is on the actual website of Dayz.
Oh this game is awesome, though i haven't played it in months
I believe my character still has 50. Cal sniper, NVGs, Camo etc
Though chances are when i logg in it'll be gone, most major updates wipe your shit.