In big friendly letters: DON'T PANIC
You can download the last version of MC20, MC20.0.132, here:
http://yabb.jriver.com/interact/index.php?topic=99544.0That may help you get version 20 working again.
On Snow Leopard (10.6.8 ?) I'm not sure how much or how little of MC21 works. I know that 10.7 is generally recommended for MC20 and MC21.
It's TOTALLY NORMAL for a new major version of MC to NOT see the previous library on Mac. They automatically migrate the library under the windows versions, but not under Mac for whatever reason. But never fear! Library Backups are here. To save the day. (Bad super hero references in case I sound like a crazy person.)
MC20 and MC21 make automatic database backups once every few days. They can be restored at any time with: File > Library > Restore library . But first you'll want to locate your backups. They are normally located at:
[Your User Directory]/Documents/JRiver/Media Center 20/Library Backups
Just find the latest backup there and you should be able to restore it into MC21. You should then see all of your media, views, playlists, etc.
Good luck!
Brian.