Have you attempted to paint on your landscape, or fill the landscape with any particular layer? It is expected to render black until you assign a material layer to anyone of the components.
Fill Layer
Also, you have to keep in mind the size of those textures. If they exceed 2048 in any direction, they will not render correctly. Be sure you have assigned a Mapping Scale to your Landscape Coordinates node as well.
Landscape Coords
If you can walk me through the process you are following, then maybe I can get to the point where you are experiencing the issue. Let me know if you have further questions.
H