One point worth clearing up just in case (it's a common point of confusion): what JRiver Mediacenter (MC) calls a "library" is a database of information about your music files created by MC. For JRiver's purposes, the "library" is *not* your music files. So if you're trying to point JRiver at your files using the "add library" dialog it won't work, as there's no "library" there, just your files. You need to use the File-->Library-->Import option to bring your files into your empty library, not the "add library" option. The import options won't move any files, they will just tell JRiver where to look for files so that they can be indexed into the "library" database.
If you know all that, and you actually mean that you have a JRiver database library at a certain file location but JRiver won't recognize it, the easiest way to migrate a library is to create a library backup in whatever version of JRiver MC you used to create the library, and then restore the library backup in the new version of JRiver.