I have done a lot of reading about DE and goats. I have not had a chance to experiment myself yet, but this is what I have found.
DE kills by contact with the worms, so you have to take enough of it, take it long enough, and also remember that it will only kill the worms in the digestive tract. Any other worms, such as lung worms, will need another dewormer.
Amount: For Nigerian Dwarf's you need at least 1/4 C a day for 3-5 days per goat. You will need more for larger goats, however the breeder that I am getting this information from has only Nigerian Dwarf's, so I don't know how much, I guess you could figure it out by weight, the average Nigerian Dwarf should weigh around 75lb.
ETA: The breeder that I read about this just mixes the DE up in with her damp grain and her goats eat it that way. I have tried that and my goats will do everything they can to avoid eating the DE (including not eating the grain) so I will have to figure something else out.