The [mappress] shortcode doesn’t have anything to do with auto-generation of the maps. But they don’t get generated until a post is saved or published, so maybe it’s a timing issue?
Try this: go to a post where you have an ‘address’ custom field and publish it – you should see the map generated, without needing to put the shortcode in the post.
The shortcode doesn’t take an ‘address’ attribute because you would then need to geocode that address every time a page is displayed. It would slow down your site and you could hit the Google usage limits (see the docs for details).