Hi Adam,
Organisations should be added automatically to the drop-down when you create a new org. as here:
https://help.totaralearning.com/display/TH13/Organisation+items
You can add a new org. by navigating to here:
YOUR TOTARA/Site administration/Organisations/Manage organisations/your organisation framework
Let us know if anything is unclear, or it doesnt work.
cheers,
George.