There are two ways. One as i said by editing /openfire/conf/openfire.xml (uncommenting authorisedUsernames or authorisedJIDs tag and putting usernames or full JIDs delimited with commas).
Easier way for you would be to click on a user in the Users list, click Edit Properties button and check "(Grants admin access to Openfire)" checkbox - Save Properties. Then this user would be able to login to Admin Console and administer the server. Note: he will get full admin rights.
