For the custom fields: if you provide both address and lat/lng, the plugin will store the address but it won’t use it for geocoding. If you provide just the address, the plugin will geocode it.
You can test you’ve set everything up correctly by entering the custom fields manually in a post and publishing it. You should see a map named ‘automatic’ is generated. Note that fields starting with underscore ‘_’ are hidden by WordPress, so you’ll need to pick a name that begins with a letter like ‘lat’ or ‘lng’.
The iconid is either a standard icon (‘red-dot’) or the name of your own custom file (‘myicon.png’). There are sections in the documentation about custom icons and iconids.