I'm attempting to upgrade from version 1 (which I was using on a custom post type). I didn't see the migration script you referred to on the settings page. I took a peek at the plugin and discovered it was hidden as the plugin did not detect any legacy content (I forced it to display and ran it anyhow - without success). I know the content is there, when I downgrade the plugin it shows up again.
As suggested, I also tried creating a new instance called 'attachments' (with the correct CPT specified), but it just shows up on the edit page without any content in it.
Any Thoughts? I love this plugin and want to upgrade!