su is a utility that is used to switch users (in a console), by default it switches you to root and many people new to unix like OS's think this is the "root command".
When you installed, you should have been asked to set up an administrator password (the root account), and then a personal account with a username and password. Is this what happened?
To configure cups you need to use the username root and the admin password.
Sorry if this is what you're already doing!