This method will let you not only install the main DayZ mod but also the community maps like Namalsk and Taviana (and every other map)
1. Install Arma 2
2. Install Arma 2 Operation Arrowhead (NOT BETA!!!)
3a. Run Arma 2 and close it once at the main menu(Important!)
3b. Run Arma 2 Operation Arrowhead and close it once at the main menu(Important!)
3c. Run both steps 3a and 3b from their .exe as admin. \Steam\SteamApps\common\Arma 2 and \Steam\SteamApps\common\arma 2 operation arrowhead are the default paths.
4. Download and install DayZ Commander http://www.dayzcommander.com/
5. Run DayZ Commander and check the two boxes in settings "run with steam" and "replace files so steam works"
6. Hit install/update and download the Beta files and the main DayZ mod and whatever extra maps you want
Now you can use the filters to find a server with the version of the mod you now have. remember this is a mod its not ment to be super easy to install, but once you get it working its worth it.