This is Likely a configuration permissions/ownership issue or a corrupted Settings file.
Do
not use sudo to run MC, you can end up creating settings files that can't be written using sudo.
If you think you need sudo, you probably just need to add your user to the
audio user group instead.
You can correct ownership issues by doing this from a terminal prompt (as your user, not root):
cd ; touch ugidfile ; sudo chown -R --reference=ugidfile .jriver
Then reenter the license key either using the box that comes up when MC runs, via Help->Registration Info if you are running in trial mode or by using the command line method in this post
http://yabb.jriver.com/interact/index.php?topic=102427.0 (don't use sudo!).
If that still doesn't work, move the Machine Settings file out of the way and rerun MC and reenter the key. From a terminal prompt as your user, not root and with MC NOT running:
cd ; mv .jriver/Media\ Center\ 21/Settings/Machine\ Settings.ini .jriver/Media\ Center\ 21/Settings/Machine\ Settings.sav
then rerun MC which will create a new Machine Settings file. Enter your key using one of the methods above and see if it sticks.