It sounds like you’re referring to this thread. The point of the Reveal Mailbox script is to show you where the mailbox is stored in Finder, for debugging purposes. The script is not used for importing and is not supposed to show you a dialog. I doubt that it’s relevant to your situation.
Are you using a Gmail account in Apple Mail? If so, you probably need to select the messages that you want to import (rather than the mailboxes), due to the way Apple Mail stores Gmail messages.