Sorry, but Mac users are currently unable to flash the bootloader from a virtual machine. Flashing the bootloader is not strictly necessary to flash the device's OS.
Mac users , you can flash the bootloader directly using [httpshttp://wwwdl.dropboxcubieboard.comorg/ssoftware/bmhvsrapuqj30datools/PhoenixSuit_ENlivesuit/LiveSuit_ForMac.msi zip LiveSuite for Mac] and following the rest of the instructions to flash the bootloader, unfortunately you still need a vm to flash the OS.
This will delete all internal user data (such as installed apps).