DayZ is a mod of ArmA and you need to own ARMA II: Combined Operations, that’s both ARMA II and Operation Arrowhead. You can’t use hacked ArmA so if you want to play DayZ BUY it on Steam for only 24,99 €.
The easy way to install and play DayZ is a nice little tool called DayZ Updater. Download it from HERE, extract in a directory of your choice and execute.
Insert the path of ArmA OA (Steam users can find it into \Steam\steamapps\common\arma 2 operation arrowhead\), click “Check for update”. DayZ Updater will check for all the necessary files and will extract it into the right directory. Then you can click “Play”. That’s all.
This is the step by step procedure for STEAM users:
1) Download both ARMA 2 and ARMA 2: Operation Arrowhead through Steam.
2) Run the game once before proceeding.
3) Download and setup DayZ Updater
4) Update ArmA to the latest beta patch from HERE
5) Copy arma2oa.exe from C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\Expansion\Beta to C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead
6) Go to Steam and right click Arma 2: Operation Arrowhead, go to ‘Properties’, ‘Set Launch Options’, add “-beta=Expansion\beta;Expansion\beta\Expansion;CA;@DayZ -world=empty -nosplash” without quotes in the target line and click OK.
7) Start DayZ Updater – check for update to update DayZ Mod
8) Click “Play”
1) When you update ArmA to the beta patch shut down Steam
2) Always start Steam and Dayz Updater with “adminstrator rights”
3) you can add “ -cpucount=4 -exThreads=7″ after “-nosplash” in the launch properties in step 6 if you have a quad core