This works perfectly!!
Make a shortcut to launch ARMA2 Beta with Steam Overlay (Optional)
Instructions:
1) Open Steam.
2) Click the Games menu.
3) Click Add a Non-Steam Game to My Library.
4) Click Browse.
5) Navigate to, most likely, C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\Expansion\beta. (If you are not running 64-bit Windows, remove (x86) from the path.)
6) Choose arma2oa.exe.
7) Click Open.
8) Click Add Selected Programs.
9) Open your Steam Library.
10) Right-click arma2oa in your games list, and choose Properties.
11) Rename it from "arma2oa" to "DayZ".
12) Set the Target to: "C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead\Expansion\beta\arma2oa.exe" "-beta=Expansion\beta;Expansion\beta\Expansion" "-mod=C:\Program Files (x86)\Steam\steamapps\common\arma 2;Expansion;ca;@dayz" -nosplash (If you had to modify the path in step 5, do so here as well.)
13) Set the Start In line to: C:\Program Files (x86)\Steam\steamapps\common\arma 2 operation arrowhead (If you had to modify the path in step 5, do so here as well.)
14) Click Close.
You should now be able to run the game from within Steam using the "DayZ" shortcut.
Source:
http://dayzwiki.com/wiki/index.php?title=How_to_install_DayZ