In order for an Account Number to be visible all segments of the Account Number needs to be setup, therefore the OrgUnit and the Object. In addition the Account Number needs to have been setup.
1) To view whether the OrgUnit has been setup see Manage OrgUnits.
2) To view whether the Object has been setup see Manage Objects.
3) To view whether the Account Number has been setup see Manage Budget Accounts.
The Create Budget Accounts articles explains the wizard that creates OrgUnit, Objects and Budget Accounts. This include the Add OrgUnits step.
Another reason why an Account Number may not be visable is due to its security setup. Please see the How Do I Activate and Use Account Level Security? article.