Instructions
Start a new "Grand Theft Auto IV" game.
Press up on your controller to access the in-game cell phone.
Start a new multi-player game.
Defeat as many enemies as possible. "Grand Theft Auto IV" multi-player mode is essentially a melee-style battle set in an urban environment. Points are attained by killing on-screen characters. You must kill enough characters to reach achievement level two. When you reach level 10, you will unlock the zombie mode. Alternatively, during your online play, if you happen to kill a player controlled by a Rockstar Games developer, you will immediately unlock the zombie mode, regardless of actual level.
During your next "GTA IV" game, you can select a zombie outfit that your character can wear during game play.