There is a tutorial in your DarkGDK installation folder that details how to create a terrain from a heightmap. I personally prefer to make a .dbo model for my terrain and use dbLoadObject to place it in my world. If you read the documentation that is in the .chm file (it is in your install directory) you will find a fairly complete list of all the different functions available in DarkGDK.
Any 3d object can be loaded into your game using dbLoadObject, you can than use dbPositionObject and dbRotateObject to position and move your characters around in the world.
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Currently 1300+ lines of code into an "over-the-shoulder" action RPG with combat based on rag-doll physics.