The lighting of the upper cloud is fantastic, the shapes of the lower one are great, too, but this is also where the lighting issues are. Like Matt sort of suggests, to make it look really good you have the reduce the influence of the fake internal scattering. If you have been using the cloud files without adjustments to your scene, I can see where the issues are coming from. The scene that came with the clouds has been optomized for brighter light, higher sun elevation.
I don't know your exact scene setup, so I am taking a guess here:
- make the cloud color brighter, to around 0.4, reduce the scattering color to around 0.6
- reduce the fake internal scattering a bit, but you'll have to experiment as to how much
- maybe reduce the cloud density and/or sharpness some
Just guesses, though.
Hey, I haven't said yet that I am actually impressed by the scene. The whole sky and lighting are leaving a quite intimidating impression

Cheers,
Frank