... I clicked on my "house" and then on Library, Preferences and found com.apple.mail-shared.plist. I don't see one that isn't shared though. Should I use that one?
I've got both com.apple.mail-shared.plist and com.apple.mail.plist...dunno why you haven't got "mail."
It should have been there since time immemorial, and it doesn't make any sense that you haven't got it, because even if you accidentally trashed it at some point it should have been recreated the next time you launched Mail.
But then again, maybe your not having it is at the root of your issue, and if so, I haven't got a clue what comes next.
At any rate, the "shared" pref has nothing to do with your issue, so leave it be.
(The Library folder in your sidebar is apparently /Library, and I suggest that you drag it out to avoid future confusion.)
More: Upgrade???
