Under MDE, look at the Resort reservation. If you have not PIF yet, there should be a like to "Make Payment". Something like this:
https://disneyworld.disney.go.com/reservation/resort/XXXXXXXXX/payments/
That will have your resort reservation number.
At first I had that problem. What worked was:
* Open a new window and go to disneyworld.com
* Login to My Disney Experience
* Go back to the BOG site
* Log in with your email/password you used for MDE
* It should then ask you for your resort reservation number (see above reply).
We are not checking in until Dec. 22. Obviously it does not let us select anything yet and probably won't for another month. However, I had two reservations that got combined. This system is now showing a party of 9 instead of a party of 5 (one res was for 4, the other for 5) - ie everyone is listed twice.