Welcome to the forum!
Shadow passes in the traditional sense aren't supported by render layers or render elements, but we might add this in future. If you just want to colour correct your shadows, I suggest outputting the Surface Direct and Surface Indirect elements. These add together to produce the full colour of the surface, and if you colour correct Surface Indirect you're mostly adjusting the colour of the shadows (although it does change the total colour even where there isn't shadow). This is a more physically correct way of adjusting the lighting, so that's how I prefer to work. But if you really need to control cloud shadows separately from terrain shadows then unfortunately you might need to render two passes, one with cloud shadows and one without. It's possible to use Render Layers to automate the setup of these two passes (I can help you set that up, let me know if you want to do that), but it might be easier to simply disable the clouds manually before rendering the non-shadow pass.
If you need the shadow pass to composite over elements rendered outside of Terragen then of course you really need just a shadow matte. You can probably do this by rendering the terrain with a pure white surface, but be careful with light levels so that you don't clip the whites.
Oh, and there is another option. If you have something like Nuke you can render two passes, one with shadows and one without, and use a Divide operator to extract a shadow pass. The results might be a little noisy if there are difference between the two renders, but it might work.
Matt
