Btw, i'm not the one who wrote this software 
Actually, original developers are not here anymore. We are just a few guys trying to answer question on the forums and a few occasional patchers.
The screen you see is only shown when you run a setup over the already setup database. The fresh install should show a screen i uploaded (no Current password field). So, if you say it is a first setup (or you just launched the setup again on an already setup server to make a screenshot?) then it might be an issue with the permissions on /openfire/conf/openfire.xml file.
I don't know what was the idea behind the "If this is a new installation, the current password will be 'admin'." line. I can't think of a real situation when this could occur. Unless you started to setup the server, abruptly stopped this and then start again, but still want to use the same database and not start with the fresh one..
I have emphasized the parts that say that the username is admin. And that it doesn't say login or username near the Admin Email Address field. I just want to point, that the text here is not misleading or implying something wrong. Actually, if i recall correctly i have filed a ticket to add this "(username of admin)" not so long ago, to make it more clear. But it looks like we still have to deal with the perception, that if email address is present in the form and there is no username input field, then you should probably use that email as login
Well, i don't like putting the email alone on a separate screen (and it would involve more programming, though it is only jsp pages). So maybe something like "This installation of Openfire is now complete. To continue press the button below and login using admin as a username and the password you have provided:" at the last page would be sufficient to clear the confusion?