I finally figured out that the problem I was having installing newer version of JRiver MC18 had to do with a corrupted uninstall of the JRiver version of HDTracks Download Manager. I went into the registry and stripped out anything having to do with JRiver, made sure there was nothing left in the Win7 Uninstall panel, and rebooted. The install for the current version of MC18 went fine.
I went to enter my registration code in the newly installed version and I can't find it on my system. I verified my purchase of MC18 last year via PayPal but I can't find the email with the registration number. I went to the Restore pages and tried my email address, my PayPal transaction ID and PayPal email address, just about very combination I could use based on the different ways to recover the ID. Each time it came up with not finding my email address, or not even being able to find the PayPal transaction ID. I'm looking at the PayPal receipt while I'm typing this! Not sure what else to do here.
I had MC18 running fine before the HDTracks incident.