I need to use the Django admin interface, but don't know how to log in.
To access the Django admin interface, you will promote an existing user to what Django calls "staff" access. This is distinct from the Open edX staff concept.
You will grant staff access to an existing user by running a command as the web server. How you do this depends on what kind of Open edX installation you are running. The command will look like:
manage.py lms manage_user username firstname.lastname@example.org
Replace "username" and "email@example.com" with the actual username and email of the user you want to promote.