I just wanna make sure you use http://localhost:3689/FirePlay.html is that correct?
You should be presented with a login dialog box, just as if you go to http://localhost:3689/playlist.html (for the playlists manager).
If not, have you put the three files in the correct folder? (/usr/share/mt-daapd/admin-root/)
If you have, check the permissions on that folder AND the three files (they might have been messed up during copying).