Moon Project editing is fun, because it allows me to design something that most fps's don't: a vast landscape. At first, it was great fun to create my favourite game worlds to stage epic battles in, then I started dabbling in making my own levels. With Farin Hills and the Coasts of Carcoon, I would say I have been successful.
Installing Moon Project maps is a piece of cake: just unzip into your "Levels" directory (eg: "The Moon Project\Levels"). The map should then be in your maps list.
If you want to design your own Moon Project maps, the editor is sitting there in the main menu ready to be used. The game comes with a pretty good starter resource, so you should have no trouble designing your own wonderful landscapes.