As of Version 3, PlaceMaker can import detailed terrain into SketchUp. You can also use the SketchUp built-in Add Location feature to import less detailed terrain.
With the terrain surface now in your model, you can then select it and import onto it with PlaceMaker.
It is also possible to create terrain surfaces manually using the SketchUp sandbox tools.
Can I use my own terrain data?
Yes, if you are able to import and properly geo-locate your terrain data with SketchUp, you can also use it with PlaceMaker. At this time, PlaceMaker does not include any tools for importing user-terrain.