Order number you have been trying to enter is probably Orbitz's, which doesn't help you with MU. Did your confirmation not come with a Ticket number, which should be identified as such? If not, call Orbitz with that order/confirmation number, and they should be able to give you the MU ticket number. Then go back to MU and try again. Be advised that your best bet with MU may be to use the phone if you want a seat assignment.