With the default atmosphere and one cumulus cloud layer preset, putting the sun elevation at 0 gives the right sort of hue, but when the sun is close to the horizon the biggest obstacle to this effect is other clouds (which might not be visible) casting shadows onto the clouds which you want to glow.
There are ways to remove very distant clouds (perhaps using the Distance shader), but a simple way to solve the problem is to experiment with slightly different sunlight headings until you find a clear pathway from the sun to the clouds you want to illuminate.
You also need to render high detail areas (perhaps with the crop feature) to really test this well (notice in the photograph how the glowing edges of the thick clouds are very thin). If you reduce the density of the clouds it becomes easier to see the glow effect on more of the clouds.
Matt
