I have precisely zero knowledge of using the Digispark on a Mac, but maybe something I suggest will help anyway.
Yes, the board *should* have a boot loader installed already. The micronucleus firmware mimics a functionality of a bootloader for the attiny, allowing it to either self-program or run the loaded user program. The trick here is the order of events. You start the upload process, and then you connect the Digispark when prompted, as the bootloader is only active in the first five seconds of powering the Digispark up. So you only plug it in when the upload process prompts for it, otherwise it won't know to accept a new program. As far as I know, no driver is needed on linux or mac, as the OS will automatically detect the Digispark.
According to the wiki page for getting started with the Digistump, it should work ok on the Mac. However, there are some Mac specific troubleshooting notes
at the bottom of that page. It suggests if the Oak isn't detected, try using a USB extension cord or USB hub, as the ports may be recessed too much for the USB connector to make proper contact. There is also mention of the 2012 Macbook Pro requiring use of a USB hub due to the USB3 controller not being compatible.
Hope that helps somewhat.
