When you deplane at IAD, if you follow the directions to the Transfer people mover, you are using the IAD midfield option and should have selected midfield for MPC use when using MPC at IAD.
The IAD main hall option is for the arriving passengers who have Dulles as their final destination (or with the checked-in bags tagged only up to IAD). So if you were just flying CPH-IAD to visit the DC area on arrival to IAD, you probably went the main hall route and should have selected IAD main hall for MPC use.