su not working on mac os x?
No need to make up a root password. Try
sudo su
and type your user password.
Others have mentioned the details of the root account. However, you don't even need su. Just use sudo -i and you get an interactive root console anyway.
For su you need to enter root password. For sudo you have to enter your own password and you should be listed in sudoers list. Since you have not set root password or root password is different then your password, su is not accepting your password.