Look here:
http://www.disboards.com/showthread.php?t=3159118&page=173
page 173:
"Originally Posted by SPOERLX3
Meriweather. You must have the 12 digit all numeric reservation #.
Here are a couple of ways to find it:
Login to the Disney website.
On the My Disney Experience tab, click "My Itinerary"
Scroll the calendar to your check in day.
On the page, place your cursor on the line that says "View Reservation Details" and right click.
Scroll down to "view page source"
Then I used the "control F" feature (just hold down control and the F key>it will bring up a "find" box>type in the word "reservationnumber" to find reservationNumber value and the 12 digit code that starts with 44xxxxxxxxxx.
OR, if you can't find it like that, I was going into my account to make a payment and found it there.
Log into your account on disneyworld.com
Hover over MyDisneyExperince till you see the options
Click on My Reservations and tickets
Under "Disney Resort Hotel Reservation" you will see your reservation details
Click on "view details" at the bottom of the reservation details
A box will pop up that will show more details on the left and your travel party avitars on the right.
At the top right under "Reservations for X", you will see "balance due (due by XXXXXXXX) $x,xxx.xx USD Make a payment"
Click on "Make a payment"
Now, look at the URL web address.
It will look like this:
https://disneyworld.disney.go.com/reservation/resort/44XXXXXXXXXX/payments/
This (12 digit # I have underlined) is your reservation number. It will start with a 4. "