To import your Facstaff Archive into Apple Mail, in Apple Mail select File>Import Mailboxes:

Then choose 'Files in mbox format' from the list of choices, click Continue and locate and select the mbox file or files from your Facstaff archive:

Once imported your mbox files will be under the 'Import' folder in Apple Mail's left hand pane:

Please contact your Local Support Provider if you have any trouble.