Do you have to start on a mac ISYNC to register you BB before you start loading up BB desktop for mac?
What OS do you have on your Mac? DM only works with Mac OS X v10.5.7 and above (Only 32-bit mode is supported on Mac OS X v10.6) and BB OS 4.2 and above.
If you are trying to install the OS on the BB using the Mac then you can't. Need a PC for that unless there is an official software release from your carrier.