Ok, let's back up and start at step Zero.
0. Do you have a license for MC20 or MC21? All recent (last 2 or 3 months) licenses work with both.
1. If you have a license that's good for MC21, you should go get MC21 from the top of the MC21 forum:
http://yabb.jriver.com/interact/index.php?board=45.0Otherwise, download the latest MC20 from the top of this forum. That should be 20.0.132.
2. When you download MC20 or MC21, you'll get a .DMG file. DMGs are kind of like a CDROM. They are a container that has files inside the container. With a CDROM, you put it in your CD drive and you get to see the files inside. With a DMG file, you double click it and OS X mounts this container as if it were a CDROM and opens it up for you to see the files.
When the DMG opens, you'll see an icon for MC, and an icon for the Applications folder. You might have to make the window bigger to see all of both icons.
3. Here's where you actually "install" MC. Drag the icon for MC over the top of the Applications folder until you get a green + sign, then let go. It will copy over to the Applications folder and show you a little progress bar.
4. Time to license it: Try running the MJR file again. It might not work, as your situation is pretty unusual. If it doesn't, MC will launch and say it's expired. If so, go to Alternate Licensing below. If it worked, you should be able to do Help > Registration Info and see that it is correctly licensed.
Alternate Licensing: If running the MJR file doesn't work properly ,MC will probably open a dialog that says it is expired. When that happens there will be a button that says "restore license". Click that button and you'll get a dialog box asking for your license key. That's the original key that was sent to you when you purchased MC. Go find that in your email. Highlight it and copy it. Then paste it into this dialog box. Then press OK and it should license your copy of MC correctly. Verify with Help > Registration info.
Good luck!
Brian.