How do I set up multiple operating units? I understand how to enter expenses, and have begun to do so, but I don't see how to record expenses to multiple accounts?

If you have figured out how to split up the entries for your 'operating units' - perhaps by class, or location, or job - then you are already done. 

Using these methods avoids the need to have separate gl accounts (categories).  You would then run the appropriate reports - such as: P&L by class / location / job