Install DayZ the Hard Way: Use Nothing But Your Bare Hands
You really should use the DayZ Commander because it makes the process so drop-dead simple, but incase you can't get it working for whatever reason or just like doing it the old-fashioned way, here is a quick step-by-step rundown of how to install the mod manually.
Steam Version of Arma II: Combined Operations
1.) Install the latest Arma II beta patch. Going forward you’ll need to manually keep an eye out for new versions or expect to run into issues finding servers.
2.) Download the DayZ Mod files, and decompress the .zip file into your C:\Program Files (x86)\steamapps\common\arma 2 operation arrowhead\@DayZ\Addons . If you aren’t using the default directory for Steam you will need to modify the path accordingly.
3.) Open your Steam games list, Right Click Arma 2: Operation Arrowhead, and select Properties.
4.) Under the General tab, click Set Launch Options, and paste the following string into the box without the quotes. “-mod=@dayz”

5.) Open File Explorer and navigate to “C:\Program Files (x86)\steamapps\common\arma 2 operation arrowhead\”. Find the file named _runA2CO.cmd, Right Click and select Open With then Notepad. Depending on your version of Windows, you might also see an Edit option when you Right Click the file. This works just as well.
6.) Click Edit along the top, followed by Find. Search for “:run” , and add “;@dayz” to the part of the line where you see “mod=%_ARMA2PATH%;EXPANSION;ca”. Remember to ignore the quotation marks above. Close and save the file.
7.) Double click runA2CO_beta.cmd to run DayZ. You’ll only need to use this approach the first time. From now on you can launch via Steam by double clicking on Arma 2: Operation Arrowhead. Make sure you see the DayZ icon in the upper right.

8.) Click Multiplayer, and find a server that matches your version number. The filter options are much weaker than those found in DayZ Commander, so you might need to hunt around a bit to find a compatible host.

Non Steam Versions of Arma II: Combined Operations
1.) Download the latest beta patch and double click to install.
2.) Download the DayZ Mod files, and decompress the .zip file into your C:\Program Files (x86)\steamapps\common\arma 2 operation arrowhead\@DayZ\Addons . If you aren’t using the default directory for Arma II you will need to modify the path accordingly.
3.) Open File Explorer and navigate C:\Program Files (x86)\Bohemia Interactive\ArmA 2 Operation Arrowhead. \”. Find the file named _runA2CO_beta.cmd , Right Click and select Open With then Notepad. Depending on your version of Windows, you might also see an Edit option when you Right Click the file. This works just as well.
4.) Click Edit along the top, followed by Find. Search for “:run” , and add “;@dayz” to the part of the line where you see “mod=%_ARMA2PATH%;EXPANSION;ca”. Remember to ignore the quotation marks above. Close and save the file.
5.) Right Click runA2CO_beta.cmd , and select Create Shortcut. Move and rename your new shortcut to whatever works best for you, and go ahead and launch the game.
Click the next page for the troubleshooting section