Where do I enable BusyContacts Mail Assistant in Apple Mail?

Enable BusyContacts Mail Assistant in Mail > Settings > Extensions. Current BusyContacts releases use this Apple Mail extension to capture email activity.

You also need to enable Capture emails viewed or sent in Apple Mail under BusyContacts > Settings > Integrations > Mail. Follow Adding Mail for the complete setup and capture limits.

The old Manage Plug-ins… button belonged to the legacy Apple Mail plug-in, which is no longer supported. Its absence does not indicate a problem with Mail Assistant. There is no need to restore that button or run Terminal commands to enable old plug-ins.