If you don't have an adjustable VGA to Mac adaptor, I recommend you get one. You'll note that I said above that the 6100 DOS compatible doesn't support the extended sense codes. ONLY on a Macintosh power on. We mention this fact in the read me for the 1.5 update. So power down and re-power up the Mac when you change monitors on the DOS card.
Macintoshes do things quite a bit differently.
Therefore that monitor pop-up in the PC Setup control panel is permanently turned off when you're using a PCI PC compatible card. There's no need to select monitors, if the card can sense every conceivable monitor. Best that you shut things down, connect everything the way you want it, and power up the Macintosh. The DOS compatible cards may have difficulty with other monitor types. Make certain your PC Setup pop-up menu is set to the correct display type.