1) Recommended: When you're creating a Single Label - on Step 2, type the address as you're creating your live label. Check the box to save the address to your Address book. After you successfully create a label with that address, it will be added to your address book.
2) You can manually add addresses to the address book one at a time from Management --> Address Book. Fill out the required address fields and click Save.
Just be sure you are saving a good address in the appropriate format so you are successful using it in the Single Label interface.
This is also the page from which you or any sub-user can edit and delete addresses.