Gee, it's been a while since I did this.

Use the NetInfo Manager (in /Applications/Utilities). "Unlock" the access, and the system will ask you for YOUR password. You can then go th the columnar display, click on users, and the list of users will appear in the 'next-right' column. Click on root, and all the info on 'root' will appear in the bottom window. Double click on the field to the RIGHT of the 'password' label, which is the encrypted password for 'root'. CLEAR that field, and 'submit it' by hitting 'Return'. Then hit <Command>-S, to SAVE this new data. You can then open a terminal window, and "su -". A password will be asked for, and the 'Return' will be it. Change the root password to something secure by entering: "passwd root". This will NOW be the 'root password', and you will be able to login at the "login window" AS root, and "su"-ing in a terminal session will work as expected.