I must be missing something or not doing something properly???
Yes, I don't think you understand the concept. You should not care where the files were installed, they are internal data that Basecamp uses. The instructions in red said to install the mytrails map: http://www.gpsfiledepot.com/maps/view/53
Did you do that? It should appear in the Basecamp dropdown menu. I would then un-install any other maps you previously installed (they are removed in the control panel like any other Windows program, it varies depending on what version you are running).
Now you can install whatever map you want to use. After doing that, you should have the Global Map, MyTrails and the other map(s) you installed available from the dropdown menu. If that is not working, not sure what else to suggest.
BTW, there is no way that anything would be installed on your GPS or a SD card yet. You must first properly install the map sot that it's visible in Basecamp, then you use MapInstall to choose a section of the map to send to the GPS or SD card.