I just bought an MP750 and set it up a couple minutes ago. I went thru these instructions before connecting the printer to my computer, and the print properties page lets me select Printable Disc as target media. But there were some differences in the setup behavior...
Enter service mode, use Right Arrow to get to #5 Type, press OK, the first thing I saw is "CYGNUS-1". Hitting the Arrow showed "CYGNUS-2". It alternates between these two choices. I toggled it to CYGNUS-2 the first time through this procedure; I probably should have left it alone. Later I went back and set it back to CYGNUS-1.
Keep in mind I don't have the mp750 nor the mp780... so folks like your self are going to have to explain these menus to us.
CYGNUS-1 : MP750
CYGNUS-2 : MP780
Clearly your menu is different from my mp760, I don't have an option for Cygnus under type that i'm aware. And the only difference between your mp750 and mp780 is the fact that you don't have a fax modem installed on your printer. I just checked my mp760 and under #5 type there are only the choices of countries and standard all reached with the up and down arrow... selected by OK or service menu exited by pressing stop/reset.