I don't work for JRiver, I'm just trying to help you out with this. So I don't know what kind of license you do or don't have.
When you get your license Key, it will tell you what kind of license it is. If you still have the email that came in, that would be a clue. But the Key isn't enough. You need a license Install Key. To do that, you do what JRiver calls a "license restore". Go to the restore page, fill out the form, and it will email you a new Install Key File. These Install Key Files are good for 2 weeks and can be used to activate JRiver MC on several computers during that time. (Like if you had a computer at home, one in your home theater, and one at your office that you wanted to license, you could use this file to activate them all within a 2 week period.)
After the 2 weeks is up, the Install Key File won't work anymore, but the computers you activated with it, will continue to be licensed. I hope this process makes sense. I had a hard time figuring it out at first!
Here's the restore page where you should get started:
https://rover.jriver.com/cgi-bin/restore.cgiAlso note that when you get the Install Key File via email (after requesting one on the Restore Page), it will tell you what type of license it is. So you can see if you have a Master License, or some other kind.
Good luck to you.
Brian.