First of all, you'll have to enable the root user. This is done in NetInfo Manager. (You have to be logged in as a admin to do this, I think).
1. Select the menu Domain/Security/Authenticate, and type in the admins password.
2. Select Domain/Security/Enable Root User. I think you will be asked to enter the root password here.
Then you'll have to change the login window, so you will be able to enter a user that's not in the list. (We're talking 10.1 here, in 10.0.x you don't have to do this). There's two ways to do this, either check the "Show other users" option in the Login preference pane, or select to display the login window as "Name and password entry fields".
I think that's all. Now you'll be able to log in to Aqua aswell as Darwin as root.