1. Buy ArmA II: Combined Operations (includes ArmA II & Operation Arrowhead add-on)
2. Install and run the game once
3. Install Beta Patch:
http://www.dayzwiki.com/wiki/How_to_...MA2_Beta_patch
4. Download and install Six Updader/Launcher
http://www.six-updater.net/p/download.html
5. Run Six Launcher, select a server and play.
Apparently, there is a way to download ArmA II Free, which is sort of a light version of ArmA, and buy just the Operation Arrowhead add-on. So in short, you can't pirate this.
For more detailed info:
http://www.dayzwiki.com/wiki/How_to_install_DayZ