The readme said to put the snoopy files in the "scenarios" folder - which is in the "Conquests" folder. Then, when you get ready to play a new game, instead of going to "New Game" on the main menu, go to "Civ Content." There you will have a list of scenarios and scenario maps. Select the snoopy terrain one and it will send you to a world selection screen to proceed as normal.
I have read that it is not safe to mess with the default editor file that the game runs on. So in other words, it might not be safe to simply save the snoopy terrain under the name of the default editor. I have not tried to- instead I do as explained above. I have, in the past, tried to make changes to the default editor, simple ones, such as changing Civilizations' colors. I get lock-ups and other odd things.