On a fresh install of Openfire 3.8.2, the documentation implies that the admin user's email address and password will be what you use to log in during the final step when you are redirected to the admin control panel.
Upon entering these credentials, I get an authentication error. Restarting Openfire as suggested in many similar error reports does not make any difference.
In order to log in as the admin user, I had to enter admin / admin as the username and password.
Once in, I am able to change the admin password, but not the admin user name, as per the final comment here: http://community.igniterealtime.org/message/185628#185628
This is a fresh install using the RPM on Fedora 20.