There IS a spot for it. OK disneyworld.com is having some ISSUES right now at this moment, so I have to go by memory. Log in, go to My Reservations, go to find your reservation (put in res number, name etc), then there is a Ground Transportation link, click that, and it should show you a page to add DME.
Was I able to use it? No. For one of three reasons. One, maybe it's just a decoy put there to frustrate us, LOL. Two, the CM said that my email address and phone number weren't on it and that keeps a person from getting the info through. Three, what I believe is the case, the page desperately NEEDS you to book both to and from the resort (or from and to the airport)...and I wasn't...we have a very split stay, and I was only booking TO the resort on that reservation, but there was no spot for that.
(however there IS a page on the DVC website for adding DME, or possibly simply requesting that they do it, and I did that for getting back to the airport)