I'm in the same boat as well. Someone said it's the number under your magic band on the mb page in MDE, and I see a 12 digit number under my MB for a previous trip, but the new MB's just show as "pending". The 12 digit number may not show up until the MB's are finalized and shipped. I've looked through my confirmation emails from Disney, and none of them contain a 12 digit number, either....only the 8 digit.
Of course, out of curiosity, I called Disney this morning and asked a cast member about getting my 12 digit reservation number. She told me that the 12 digit numbers are for room-only reservations, and the 8 digit alpha-numeric is for packages. Not sure if that's accurate or not.
On the plus side, it appears that the 8 digit alpha-numeric confirmation also works, so we should be good to go.