Well, the mailing address for DC is DC Comics, 1700 Broadway, NYC, NY 10019. So, if you were writing a letter to Dan Didio, for instance, you would address it:
Dan Didio DC Comics 1700 Broadway NYC, NY 10019
So that goes on the envelope, name included (and then of course standard letter-writing says you should address the person you're writing to in the letter itself).
Last February I sent a couple letters to Dan Didio, Ian Sattler and Paul Levitz, and addressed them that way. (Mind you, I have no idea if they actually received them, as I never got a response, but as far as I know that should work.)