Did you get the email with your license code and the .mjr file? If so (and you have the .mjr file), on Linux make sure MC is closed and then open the Terminal and type the following in;
mediacenter23 /RestoreFromFile pathtothe.mjrfile
For the
pathtothe.mjrfile part you can simply drag and drop the .mjr file into the Terminal to get the path in (please keep in mind you need a space between the
/RestoreFromFile and the
pathtothe.mjrfile part in addition to the space between
mediacenter23 and
/RestoreFromFile). I suggest moving the .mjr file to the Linux desktop if your desktop environment supports it. If not, from the normal Downloads folder (in the Home directory) should suffice.
That should do it, you should get a message popped up telling you the restore was successful. Reopen MC and go to Help > Registration Info... to verify your MC is registered. If so, congrats!