Hmmm.
Perhaps you will have to find a Windows machine and attempt the update from there instead. Not sure why the USB connection would fail with the utility unless there was a problem with the printer. Normally as long as the printer is on the READY screen the utility will find it and allow you to update.
You could also try to cold reset the printer and try again with your Mac. Perhaps there was some temporary issue on the machine that the reset can clean up.