When you use the -c flag to su, you need to put the command in quotes:
su root -c "cat /var/log/messages"
This should prompt you for a password, and if you enter it correctly it should work.
There is only one super-user account, and that is root.
I have never heard of the "super" command.
Maybe it would help to post what's going on in your terminal, IE:
user@hostname:~> tail -n 1 /var/log/messages
tail: cannot open `/var/log/messages' for reading: Permission denied
user@hostname:~> su root -c "tail -n 1 /var/log/messages"
Jun 23 11:08:32 hostname su: (to root) user on /dev/pts/0
user@hostname:~> su -
hostname:~ # tail -n 1 /var/log/messages
Jun 23 11:08:44 hostname pure-ftpd: (ftpuser@IP) [INFO] Logout.