Hi,
mjr files are what JRiver calls "temporary Install key". Those files are valid for 14 days after they have been issued.
Normally, you install a mjr file by going to Help->Install License... where you have the option to enter the path to your mjr file. That's the behaviour on Windows.
But on Mac it appears to be a bit different for now.
When going to Help->Install License... you have only one option: enter your permanent registration code (the one JR gave you upon your purchase) see attached screenshot. When you do so, you activate MC Mac and issue a new mjr file that's emailed to you.
This 'mjr' file could normally be used to activate MC on another machine you own by either clicking on it or entering it somewhere in MC.
But, in fact None of those activation procedures (based on mjr file) seems to be working now. That's what I noticed on my 3 Macs.
It's probably a temporary (alpha) behaviour.
Cheers,
C.