I don't know anything about Mac, but when I had that trouble on
windows, I always needed to remove the key, and then reboot the
computer. I waited for the
computer to fully boot up and settle down. Then I plugged in the key.
I assume it's a flash drive. If it's being seen, there should be a green light on it.
Again, macs may have a different type of dongle
Also, do you have more than 1 port to plug into ? Maybe your putting it into a different port than it's used too, and the port doesn't work