The custom field names will be whatever field was saved by the form. You can check the database table wp_postmeta for a post that was saved to find the correct names – they usually don’t have spaces in them.
If the address isn’t valid, or your server is over the Google geocoding limit, you won’t be able to geocode. In that case, the plugin will write a custom field called ‘mappress_errors’ with the error message in it.
The before/after options are a convenience. You can put the shortcode [mappress] anywhere in your post.