So you bought a non-Steam version of ArmA II? Installing the alpha version of DayZ is no easy task, as it is an alpha. This guide should help those who bought through Retail, Bohemia Interactive’s store, or GOG.com (the version this guide is based on).
To be clear, DayZ mod requires ArmA II: Combined Operations which contains ArmA II and the expansion pack, ArmA II: Operation Arrowhead. You could also use ArmA 2 Free + Operation Arrowhead to play DayZ, but it isn’t recommended (via DayZ Wiki)
I’ll be covering the GOG.com version of the game, but it will apply to the other Non-Steam Versions of ArmA II to help get DayZ installed.