Instructions
Go to GTAGarage.com (see References for link). If you have an account, log in; if not, it is a good idea create one, though not absolutely necessary.
Click "Major Modifications."
Browse the list until you see "GTA 4 Zombie Apocalypse." Click that link.
Click "Download Mod" on the right-hand side of the screen under the screen shots in the "Files" box. It will open a list of files.
Download the most recent file as this will be the most complete. Once you click the file, a dialog box will open confirming that you want to download it.