It only lets me choose 600 dpi. I will also note that I have not installed any drivers as none are available for 10.10.
Go to Solution.
Use the Phaser 5550 driver that comes with 10.10 and 10.11 if you want the 1200DPI selection.
It isn't officially supported, but since the 5500 has not been sold since 2008, and not supported or developed from since 2012, you have no real options otherwise.