Creating a simple terrain
From Mod Wiki
For now a flat terrain is fine to drive and run around on. The example map already has a flat terrain set up. The terrain mesh was added to a map via the Terrain Editor. Pick the Terrain Editor inspector and right click on the empty left hand dialog. Then choose New > Tree... Then choose the following terrain model "models/terrain/sdk/etqwmap.lwo". Some guidelines on making your own terrain model can be found on this page Terrain