I did not mention the version of windows, and I certainly understand that. I have no problems with winMC key since 10 version).
I can not use the MAC key with the MAC version. The only discrepancy - MAC Player 18, but the key "JRiver, Inc. Media Center 19 Mac" - but its not my choice, I just click Buy from MC18 MAC and pay.
Update your Mac version to the latest.
and not registered MC Mac 18.0.218...
Latest, right?
PS my mac is power on and have Internet access