There are a lot of topics discussing this over the years, you can search for here on the forums. The best method is to just apply your terrain to a height field generate, generate it, and then use it's colour output set to shade by height. Then you can stick that shader into a surface layers luminosity. You don't need GISD and other features of Terragen post processing.
Then you just setup a camera pointing down at the terrain, set to orthographic, and render.
However, with the demo, you'll not be able to do more than a 512x512 export, I believe (due to resolution constraints), and I don't believe there is orthographic camera perspective but could be wrong.