Which version of Openfire are you using? In the current 3.10.0 builds this part is missing, but 3.9.3 should still have it. Anyway, you can try adding this between tags, so it should look like that:
< /adminConsole>
< admin>
< !-- < authorizedUsernames>< /authorizedUsernames> -->
< !-- < authorizedJIDs>< /authorizedJIDs> -->
< /admin>
< locale>en< /locale>
< !-- Network settings. By default, Openfire will bind to all network interfaces.
Alternatively, you can specify a specific network interfaces that the server
will listen on. For example, 127.0.0.1. This setting is generally only useful
on multi-homed servers. -->
< /admin>
or
< admin>
< !-- < authorizedUsernames>< /authorizedUsernames> -->
< authorizedJIDs>user1@servername< /authorizedJIDs>
< /admin>
Note, that you have to uncomment selected tag by removing < !-- and -->. Your servername is in System Properties “xmpp.domain” field.
Then you should save this file and start Openfire. Then it should allow this user to login to Admin Console as admin. Note, that this part will disappear from the openfire.xml part after Openfire starts and imports this information into database.
ADDED SPACES IN EVERY TAG (AFTER < SYMBOL) AS FORUMS WON"T SHOW THEM OVERWISE, THE SPACES SHOULD BE REMOVED