As long as your arrival date is correct, you're fine. I'm sure that if you provided them with your flight info, they'll have it in their system.
Now, as for your luggage. Is there some reason you will be lugging it with you to the DME checkin desk? If you use the yellow luggage tags that came in your packet, you won't need to worry about your bags! Just put a yellow tag on each piece of checked luggage, then forget it until you get to your resort room, where the bags will 'magically' arrive, several hours after you do.
The DME checkin desk is on the bottom level of the terminal, Side B. If you get your own luggage, you will have to go to level 2 to baggage claim, after arriving on level 3. Then, you will get your bags, and then head down a level to level 1 where DME is located, on Side B. Your bags will be put underneath the bus and arrive at the same time you do. Your bags will never get to the 'baggage carousel'..the bags are taken out before they make it there, then taken on a different truck to the various resorts.
Anything you may need for the first hours at WDW should be in your carryon bag. I always take a change of cloths, swim suit, all my park passes, and resort confirmations..that kind of thing. Electronics, meds..they should all go in your carryon as well.