Welcome to BYC! There is plenty to learn here.
Why would you want to use DE in composting? It isn't directly necessary/beneficial to the composting process. It is primarily silica which won't contribute to the organic breakdown process.
Depending on the type of composting, it can actually be a negative. Bugs can help to break down organic matter and DE will damage them. I did read that DE will not harm worms if you are doing worm composting.