Try this, instead:
1. Create a new folder on your Desktop and name it Pine Mail.
2. Inside that folder, create another folder, named Pine Messages.
3. Place the Pine .mbox file inside that inner folder.
4, Use Mail's Import Mailboxes… feature, choose "Other" from the list of options it gives you, then navigate and choose the Pine Mail folder; don't go any deeper in the folder hierarchy. Mail should find the .mbox file inside and present a list of mbox files to import, with that one already checked. Choose "Continue" to finish the import process. If successful, Mail will show all the imported messages in a folder named Imported, from which you can then move them to wherever you like.
Mulder