This is a guide on how to properly join vanilla DayZ Mod servers.
  Some players report running Steam as an Admin solves a lot of issues. 1. Download Arma 2 from Steam. 2. Start Arma 2. 3. Download Arma 2 OA from Steam. 4. Start Arma 2 OA. 5. Download Arma 2: DayZ Mod from Steam. 6. Start Arma 2: DayZ Mod. 7. Press Multiplayer and make sure it says "Internet" in the lower left corner. 8. Double click on the server you wish to join.  
If you get kicked off the game when joining a server: Delete any copied in Addons folders in your Arma 2 OA folder: (There is suppose to be an Addons folder in the Expansion folder, don't delete that.) If you get "Session Lost": Make sure you are launching DayZ Mod from Steam, not Arma 2 OA.