Not to sound arrogant, but stick to PolyCount or GameDev, UDK's forums aren't exactly a bustling hub of helpfulness in many cases.
As for the question, most clouds are made to fake Scattering properties, like Skin, Snow, Atmosphere, etc. Crysis does a good example of this.
Basically, create a Fresnel, and have it feed on an inverted Normal of your clouds, doesn't matter if you take Light or Camera calculation, both are valid for Fresnel in this case.
The edges of the cloud in question should be light and the inner parts dark.