Instructions
Press the "Up" controller button to pull up the on-screen cell phone.
Highlight the "Multiplayer" option and press "X" to begin online play.
Kill everyone you see. The only way to unlock the "zombie" costume for "Grand Theft Auto IV" is to either reach a ranking of "2" in the online world or kill an employee of Rockstar, the company that makes the game. Rockstar employees are not marked as such in the game, so you'll never know if you've killed one until you already have. The best chance at unlocking the zombie costume is to try to reach a rank of "2" and hope you kill a Rockstar employee along the way. When you complete either of those two tasks the "zombie" costume will unlock.