Instructions
Go to the GTA Garage website and register if you don't already have an account. If you do have an account, log in and go to the download page for the "GTA 4 Zombie Apocalypse" mod.
Click the "Download Mod" button on the right side of the screen, under the in-game pictures of the zombies. This will open a download menu in the same box.
Click the "Download" button to the right of the mod marked as having been uploaded on July 7, 2010. This is the full version of the mod. Click this button to download the mod.