1- Launch arma 2 through steam installing direct -x a bunch of stuff and battle eye.
2- Close Arma 2
3- Launch Operation Arrowhead updating the game and installing the same stuff as arma 2
4 - close operation arrowhead
5 - Download play with six : http://play.withsix.com/download
6 - Launch play with six (say yes to any popup messages)
7 - Press the engine button on the top left (Right to the play button)
8 - Press Game Settings on the bottom left of the screen.
9 - Change the mods folder parameter on the right side of the screen to:
C:\Program Files\Steam\steamapps\common\Arma 2 Operation Arrowhead
10 - Press play again then the No mod selected square.
11 - Select the DayZ mod witch should be somewhere on the mod list.
12 - Press the install button ( When the mod finishes installing and 
appears install again do it again thats just a way of updating the mod)

ACE(required by AST server)
*Repeat steps 10, 11(choose ACE) and 12