I think this next bit should be a sticky, because it comes up a lot, and I've fallen victim to it on many occasion.....
No object imported into Terragen can have a space character in any of it's materials names, including directory paths, otherwise it won't create the necessary Parts Shader in the internal network of the object, you'll get an error message, and will render out black.
Not saying it's the problem, but it's something to be aware of. Oshyan explained really well why it happens, and understand the difficulty in getting over the problem in software terms.
