It's not clear, but it may be that you are highlighting a message, not a folder. At least, that's the view I get when highlight a message.

You can't import to an IMAP folder, but you can to Local Folders. Or, just open the profile folder (from Help/Troubleshooting Info, Profile Folder, Show in Finder), close TB, copy the mbox into Mail/Local Folders, remove the .mbox extension, restart TB, look under Local Folders in the Folder Pane.

Moving the messages from Local Folders to an IMAP folder is more involved, best done in small batches of a few hundred at a time.