Rusell,

You need to set up an Operating Unit at the Accounting Set up when you create the
Primary Ledger. There is really nothing different than setting up any other functional
operating unit other than the fact that this OU is not really required by the business
but is required for some features to function.
As per my understanding you need to have one operating unit logically - Operating units
are assigned to ledgers and a default legal context.
The MO: Operating Unit profile option only provides access to one operating unit. The
MO: Security Profile provides access to multiple operating units from a single
responsibility. Hope I have been able to clarify it a little bit.
Thanks,
Arun