Authentication required to modify a printer- what's the password?
You might be affected by this bug.
Try running
gksudo system-config-printer
in a terminal window, then setting up the printer
However, if gksudo is not found, try:
sudo -H system-config-printer
Adding my username to the lpadmin group, as suggested by msanderz, worked for me. Here is how you add yourself to the group:
sudo usermod -a -G lpadmin YOUR_USERNAME