But if you did a custom install of your system, you might not have it. If you do, then go to your extension folder, open the folder called MRJ Libraries, click on the file MRJLib and get info. It will tell you what version of MRJ you have.
The only time I use MRJ is when I have Internet Explorer open. If you use Netscape Communicator, you may not need the download. There are other programs that use MRJ, but I can't name any right now.