So I was fighting with this for awhile and I wanted to post my solution because I didn't see it anywhere. I was trying to load an old library but it no longer was at the path in MC and there wasn't an option to relocate it.
So what I did was open the file User Settings.ini, which is at ~/Library/Application Support/J River/Media Center 27/Settings. Look for the following lines
[Properties\\Libraries]
0="<XMLPH version\=\"1.0\">\r\n<Item Name\=\"LibraryPath\">\[App Data Path\]Library/</Item>\r\n<Item Name\=\"Hidden\">0</Item>\r\n<Item Name\=\"NamedByUser\">0</Item>\r\n<Item Name\=\"LibraryServerAccessKey\"/>\r\n<Item Name\=\"Type\">1</Item>\r\n<Item Name\=\"DLNADeviceID\"/>\r\n<Item Name\=\"LibraryServerURL\"/>\r\n<Item Name\=\"Origin\">1</Item>\r\n<Item Name\=\"DisplayName\">Main Library</Item>\r\n<Item Name\=\"ServerPresentationPage\"/>\r\n<Item Name\=\"ServerDeviceURL\"/>\r\n<Item Name\=\"Secure\">0</Item>\r\n</XMLPH>"
1="<XMLPH version\=\"1.0\">\r\n<Item Name\=\"LibraryPath\">\[App Data Path\]DLNA Library/uuid_4d696e69-444c-164e-9d41-245ebe4f84db/</Item>\r\n<Item Name\=\"Hidden\">0</Item>\r\n<Item Name\=\"NamedByUser\">0</Item>\r\n<Item Name\=\"LibraryServerAccessKey\"/>\r\n<Item Name\=\"Type\">2</Item>\r\n<Item Name\=\"DLNADeviceID\">uuid:4d696e69-444c-164e-9d41-245ebe4f84db</Item>\r\n<Item Name\=\"LibraryServerURL\"/>\r\n<Item Name\=\"Origin\">3</Item>\r\n<Item Name\=\"DisplayName\">barkbox</Item>\r\n<Item Name\=\"ServerPresentationPage\">http://192.168.2.69:8080/</Item>\r\n<Item Name\=\"ServerDeviceURL\">http://192.168.2.69:8200/</Item>\r\n<Item Name\=\"Secure\">0</Item>\r\n</XMLPH>"
2="<XMLPH version\=\"1.0\">\r\n<Item Name\=\"LibraryPath\">/Volumes/home/Happy/Twinks/zMediaCenter/</Item>\r\n<Item Name\=\"Hidden\">0</Item>\r\n<Item Name\=\"NamedByUser\">1</Item>\r\n<Item Name\=\"LibraryServerAccessKey\"/>\r\n<Item Name\=\"Type\">1</Item>\r\n<Item Name\=\"DLNADeviceID\"/>\r\n<Item Name\=\"LibraryServerURL\"/>\r\n<Item Name\=\"Origin\">2</Item>\r\n<Item Name\=\"DisplayName\">Twinks</Item>\r\n<Item Name\=\"ServerPresentationPage\"/>\r\n<Item Name\=\"ServerDeviceURL\"/>\r\n<Item Name\=\"Secure\">0</Item>\r\n</XMLPH>"
Version=i:"3"
So in this case, I had a library called Twinks (don't ask) and the old path was /Volumes/Misc/Twinks/zMediaCenter. Because I changed my NAS server, that path no longer exists and recreating it would be a giant pain. So I changed /Volumes/Misc/Twinks/zMediaCenter to /Volumes/home/Happy/Twinks/zMediaCenter/. Once I saved the ini file and reopened MC, I was able to load the library without issue!