You can easily open & read MBOX files on Mac OS using MacUncle MBOX Viewer. The tool supports Mac OS Machines.
Start MacUncle MBOX Viewer on your Mac Computer.
Select MBOX file using Add File(s) or Add Folder(s).
Choose the requirement folders with MBOX files. And Click on the email messages listed.
Go to Search option to search emails within the MBOX Files
Enter the search keyword to "Search For"
"Has Attachments" will display all the emails within MBOX files with the attachments.