Hmmm. I am usually a Windows user, not a Mac user, but maybe...
When you first started MC21 for the first time, it would have automatically started Auto-Import after a 45 second (I think) delay. The library would have been empty until it did this Auto-Import. A MC installation is supposed to find the previous version library and use that, but it doesn't seem to do that in all cases, and the Mac install is quite different to the Windows install, so maybe it isn't intended to do that.
Anyway, first thing, are you sure you are running MC21 from the Applications folder, and not from the installation DMG? It is soooo easy to just open the DMG and run MC from within it that I did it on my first Mac install, and then got entirely confused about what was going on. So, if you are running MC21 from the mounted DMG, you need to fix the installation before anything else. That is done by copying the MC21 folder from the DMG to the Applications folder. If you need to do this, perhaps you should delete the installation file you have, since it may have had changes made to it while you used it, and re-download the installation file from JRiver.
Once you are sure that you have MC21 installed into the Applications folder, before running it, you need to decide what you want to do next.
If you just want to create a brand new library for your files on the new NAS, then start MC21, stop Auto-Import before it commences automatically after a countdown, change the file locations in Options, configure MC21's Auto-Import to watch the new NAS folders/address/whatever is needed, then run Auto-Import. The new blank library that was installed with MC21 will now be populated with your music.
HOWEVER, if you want to get your old library from MC20 back with all the information already set up in it, and you haven't uninstalled MC20 yet, then that is probably doable. There will be backups for MC20 somewhere. Sorry, I don't know where they would be exactly. Search the Mac, or around the forum to find that info. Once found, start MC21, stop any Auto-Import initiated with the new library. Then restore the MC20 backup into MC21 (File/Library/Restore Library, or Mac equivalent), change the file locations in Options, configure MC21's Auto-Import to watch the new NAS folders/address/whatever is needed, and make sure that the "Update for external changes" flag is ticked in the Auto-Import options, then run Auto-Import.
I think the second process should work fine. If I have missed something someone may speak up and let you know.
Oh, one more thing. If the first installation of MC21 created a backup of your new library before you deleted it, you may be able to simply restore that backup and get back most of what you had already done. It depends on when the backup was created.