Ah OK downloaded and Extracted. I this case the image format is a CDI file ( .CDI extension ) to handle such a file I actually had to update IMGBURN with some .dll's, but it works now and is of now consequence for MAC. Though obviously whatever Burning program you end up with for your MAC it will also need to be able to handle that format.
ANYWAY, I burned the image successfully as far as I can tell. So, exploring the resulting disc on my PC it appears as THIS:
As you can see several files. If you have but a single file ( possibly the same as extracted file NesterDC SE 3165 by cOcO! ) that means you simply copied the entire image to the disc and not burned the image contents back to a bootable Dreamcast disc.
THIS should be the files you see on the properly burned Disc. If you can get this result do same onto a CD-r, if it still does not work try a different brand CD-r (Dreamcasts CAN be picky) IF YOU ARE SURE YOU ARE BURNING TO CD-r PROPERLY BUT CAN NOT SUCCESSFULLY BOOT A DISC AFTER TRYING THE CD-r WITH DIFFERENT BRANDS TRY ANOTHER DREAMCAST IF POSSIBLE. Some will tell you about later model MIL-CD or some such Dreamcasts that were incapable of booting backups. I have 3 Dreamcasts spanning first to last production run and it would be very unlikely you'd have a Dreamcast that simply does not boot backups, I believe most people that make that claim simply burn their discs improperly.
ONCE PROPERLY BURNED THE RESULTING DISC APPEARS AS FOLLOWS ON MY PC:
HERE ARE THE FILES ON THE DISC, YOU SHOULD BE SEEING AT LEAST THE SAME NUMBER OF FILES AND FILE-NAMES:
I hope these are your results or that you can figure out how to replicate these results in the end. Good Luck To You.