Well I was going to have a go but...
 

After buying and installing combined ops through steam, the expansion (operation arrowhead) just simply doesn't work :(

This means I cannot load the game, let alone the dayz mod. Bummer :(

Really? , hope problem is temporary. Thinking of trying DayZ for Laffs :)

I had the same problem when I first installed it from steam. So I reinstalled and downloaded the mod directly from DayZ forum and it works now. Also you gotta keep up with what the current version of Arma 2, DayZ, and Battleye. Once you get the game working its easy keeping it updated with Six Launcher. Hope it helps !

Ask one of the guys in TeamSpeak to help you out. Most of them have the Steam version, and can probably assist you.
I have the DVD version myself, but from what I hear, you have to install and then run each instance of the game before anything else. ie run Arma 2 =>close the game; run Operation Arrowhead => close it down, run Steam as administrator etc.

One of the guys will be able to give you the full run through with the Steam version.

oh that is true maddog2204. You do need to start up the game before starting the install of DayZ.

AlphaGamma,
There are no laffs in Dayz. This is a serious game!

http://www.seriouscat.com/serious_cat_is_serious.jpg

Note: I hate cats, but this was the only "serious" meme I could find.

Hi guys.

Run both Arma II and ArmA II combined Operations once from your Steam games library to complete the installation of the base content.

Then follow this guide How to install DayZ (Steam version) - DayZ Wiki to install the Day Z Mod. If you've already installed Day Z, go back, remove the Day Z hook from the launcher and run both games from Steam.

Unfortunately, it's not made crystal clear in most install guides so it's an easy mistake to make but luckily it's just as easy to fix. Welcome to alpha.

Did all that, and it didn't work.

BUT! Running steam as admin did the job, cheers! (it wasn't working even without DayZ)

DAYZ IS SERIOUS BUSINESS!!!@#$!@#$

If your dayz is not working - post up with the details, we'll sort you out 100% of the time :)

You might not like some of the solutions towards the end of this process because they involve heavy hardware but we certainly will not give up unless you do first :)