I downloaded the Live USB stick image, booted from USB, pressed Ctrl+Alt-F1 and got the console screen "
login:". Now, what is the default password for "root" user? I need to load some kernel modules and mount some filesystems before installing.
Try user ubuntu without password(with empty password). As @Panther said, root account is disabled. If its not ubuntu but lubuntu or xubuntu try distributive name as user login.
Another way, if you run graphical environment press Alt+F2, and run
xterm. It will not ask for login. Then
su and do what you want.