You need to get ARMA II Combined operations. It should be 29.99 on Steam, After you get those go to
dayzmod.com and down load the Alpha mod and get all the files there.
After you have the files,
Make a folder named "@DayZ" in your Arma 2: Operation Arrowhead directory.
Inside "@DayZ" folder create another folder named "Addons". It should look like this: C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\@DayZ\Addons
Unpack all DayZ Mod files in the "Addons" folder.
Then Load ARMA II and The Combined Operation game once, then activate the mod from the expansion setting then join the game
Should be somewhat close to what i posted.