Um, no. At least on my install (710 DVD x86) linuxmce is not root. Root does not have a password, hence you cannot login or su to root. Following the ubuntu guidelines, you should perform root tasks using sudo.
If you don't want to do it this way, login and give root a password - 'sudo passwd root' Just remember this is not the intended method.
edit: I may have been confused, are you trying to login as root or just login? If just login, use the linuxmce as login.