For installing the plugin, you add it to a folder in your project named 'addons'. Then once you do that, it will appear in the Project Settings under the Plugins tab. There you will want to make sure it is checked and set to Active.
With the Terrain editor, if I recall correctly, you browse your nodes to find it, it's a 3D node. Once it's added, as GlaDOSik points out, you will want to set up your terrain size in the properties, and you will then see a surface appear. Afterwards when you have the node selected, it usually puts a toolbar at the bottom of the window that has all the terrain brushes.