Step 1
Add the games on your steam library an instal they:
1.1 - Add Arma 2
1.2 - Add Arma 2: Operation Arrowhead
1.3 - Add Arma 2: Operation Arrowhead Beta
1.4 - Add Arma 2: Dayz Mod

Step 2
Run the games in the right order until main menu then exit it:
2.1 - Add Arma 2
2.2 - Add Arma 2: Operation Arrowhead
2.3 - Add Arma 2: Operation Arrowhead Beta
2.4 - Add Arma 2: Dayz Mod (pick "Play Arma 2: Dayz Mod")
2.5 - Add Arma 2: Dayz Mod (pick "Play with Beta Patch")

Step 3
Go to official Dayz Mod website "www.dayzmod.com" and download the lastest version of the dayz mod
3.1 - Open "www.dayzmod.com"
3.2 - Click on "Download Dayz Alpha Test"
3.3 - Click on "Manual Instalation Files FR Mirror" (or the mirror its closer to you)
3.4 - Click on "here" of "If torrent isnt available for you then you can download the required files here for a manual installation."
3.5 - Look for the lastest version, ie greater version number or the last date posted and click on it
3.6 - Look for the @ file that have the words "Client" and "Full" on it and save it on your desktop
3.7 - Unrar it to a folder.

Step 4
Find and delete the olds dayz mod files to copy the new ones, overwriting it if needed:
4.1 - Right click "Arma 2: Dayz Mod" on your Steam game Library
4.2 - Click on "Properties"
4.3 - Click on "Local Files"
4.4 - Click on "Browse Local Files..."
4.5 - Delete:
4.5.1 - Delete "@Dayz" folder
4.5.2 - Delete "dayz.bikey" insite the folder "Keys"
4.6 - Copy and past:
4.6.1 - Copy and past the new "@Dayz" folder
4.6.2 - Copy and past the new "dayz.bikey" insite the folder "Keys"
4.7 - Copy, past and overwrite:
4.6.3 - Copy and past the others new files, overwriting the old ones

Step 5
Play it:
5.1 - Run "Arma 2: Dayz Mod (pick "Play with Beta Patch")
5.2 - Click on "Multiplayer"
5.3 - Look for a server that works for you with the rules that you like

Step Extra
Some times (0.00001% on steam) the game dont download it self right, and you need to "Verify Integrity of Game Cache..." if that happens you need to verity they all 4 first, then run they in the right order like on "Step 2", or you will get wrong cd-key isue. And remember, if you "Verify Integrity of Game Cache..." of "Arma 2: Dayz Mod" you will need to do "Step 4" again too.

Any question about it, just ask.