Well, you shouldn't have left the root passwd blank. Try using just <enter key> at the prompt.
However, if this is just running on a CDROM, you can't write to it anyway, so I don't think you can physically add a new user; where would it save the info ???
Maybe it saves it into RAM until you shutdown..?
I've never gone this route ; went for dual-boot, then full install. Mind you, when I started those were the only options
HTH, else wait for someone who knows more or maybe try Mint forums or google ?