I just took my first trip after buying the AP discount online. There is a website for AP users, but as you thought, you will have to turn in your voucher down at WDW to get your tickets, and your AP number will be on the back. After you get back from WDW, you can then go online to the AP
website. But let me tell ya this, you don't have to register online to get your
discounts, its just a special AP website that you look at with all the AP info available for ya. As for DDE card, there is a site on the DISboard that will
answer most questions on the AP discounts (including DDE). Since I have not posted that many replies to notes, it won't let me insert the exact address.
So just go the the first screen after you log in and take the discount tab up at the top (just under the discuss tab). Then in the middle of the screen you will take Annual Passholder Discount. Then Disney Dining. Have a Magicical day.
Mark