This addon is a function library. This means that it doesn't have any UI on it's own, but rather provides functionality for other addons. If you are an addon user and not an addon developer all you need to do is to leave this addon turned on if you have any other addons that depend on it.
Earth has not been fully documented yet. However, you can see most of the documentation by reading the .lua files.
For an example of how to use it, see Tutorial: Using Earth to Create a QuestLog.