Just enter "~/Library" (No quotes) in Go to Folder and navigate from there to /Library/Preferences/com.apple.mail.plist BY HAND.
That plist replicates itself if Mail isn't quit when it's trashed, and it will do the same if it's trashed after Mail is quit but before you restart, which is why the restart is necessary.
I can't imagine why you couldn't just drag that file from its home to your desktop; it's one of the few things that Apple hasn't prohibited yet.
