I had that problem in Beta 2 so I made sure to add a user during the LMCE install. (RC)
I could then log into the admin site with that user.
Not sure why, but it seems it asks for a LMCE user/pass combo, and not a Kubuntu one (if that makes sense)
You could possibly add a new user from the orbiter?
(I did not try this)
Bingo! I could log in with one of the users that were created during the setup (password=name), thank you. I thought I had tried that as well, but I guess not.
Last time I installed Kubuntu with the same initial username as the first user I created, that seemed to cause a (very minor) conflict, so I used "linuxmce" for the initial kubuntu user this time, hence the confusion on my part.