I seem to have the same problem. It says that my domain is available. However this is probably due to the fact that I created the account 2 months back and did not upload my site (was out of station with a connection that wouldn't even upload a 2 kb file to save a life).
Anyway, I had my username, password, cpanel details in a mail that was sent to me but I can't log in to my cpanel. However, my account username and password (for x10hosting) still works.
I tried re-signing up for an account using the same username (thought I would be able to just add a domain to my account), but it didn't work and asks me to enter a new username. I don't want to do this as I think it will just create a mess of everything for the hosting site as well as me.
Any advice on how to get around this would be appreciated.