Are you trying to run the MJR file that was emailed to you? Sometimes that doesn't work correctly. Usually when you also have MC20 installed.
But, if you open MC21, and click the "restore" button and go through the process, it should license it correctly. It will use one more restore when you do so, but it should get you up and running. JRiver is pretty good about reseting restore counts in cases like this. If you run into an issue they'll almost certainly reset your restores. So no problem.
Brian.