Read the ReadMe in the pack.
To install, extract the scn999PixiggersWorld.ztd file to the dlupdate folder, if you have it, or to the Updates folder in the main Zoo Tycoon folder, and the PixiggersWorld.zoo file to the maps folder in the main Zoo Tycoon folder.
Start the game, play a scenario/tutorial game and the game will appear at the bottom of the list. You should note that the scenario will be unavailable until all previous scenarios have been completed.