I'm using Gnome Shell (although I'm not sure whether the "Printers" app is part of that or just part of Gnome).
I go to "Printers" from the overview, click the + button on the bottom left, choose network and then see a message "FirewallD is not running. Network printer detection needs services mdns, ipp, ipp-client and samba-client enabled on firewall.".
It's not auto detected anything. There's an Address box and a Search by Address tickbox, but neither seem to do anything. I'm frustrated because I know all the details of the printer, but have nowhere to put them in. Automation is supposed to make things easier, no?
For example I know the printer is at lpd://10.67.5.3/lp1 but I can't put this in anywhere.
Is there a GUI that works, or have I misunderstood how to use it?
The Gnome Printer interface still has quite a few bugs (like the one you mentioned). To use the Printer interface that comes with Ubuntu, press Alt F2, and type in system-config-printer.
Ditch gnome's GUI and revert to the good ol' CUPS interface at
http://localhost:631
Click Add Printer and follow the instructions on screen.