There are three parts to this Zip package
1. This readme
2. The Map
3. The Texture Pack

To use the Map, open Minecraft and log in.
Click the Mods and Texture Packs button
Click the Open texture pack folder button
Copy the Zombie Texture Pack into this folder
Now go up one folder level
Open the saves folder
Copy the horizon_city_demo folder in its entirety to this folder
Now close the folder
Make sure the zombie texture pack is selected
Then go to singleplayer
Play Horizon City!
Have fun!

Alternatively, look up a video on how to install maps and texture packs (this is a default 16x16 texture pack). 