Great!
Now for the next step: shaders.
First thing you need to do is look in the folder where you exported the .obj model. Is there a file of the same name as your model, but with the extension .mtl? That is a material file, it tells TG what materials go to the different parts of your model.
If you have that .mtl file in the folder with your model, you're ready. Open the Terragen scene that has your model in it, then double click on the model loader (thats the node that tells TG which model to display) and modify the shaders in the model. Search the forum here for help on how to do that.