How to point a Dokku app at the root domain of the dokku server
As of v0.3.10, Dokku ships with a domains plugin. This lets you easily add domains to your app. By default your app is located at myapp.mydomain.com. If you want your app to be accessible via the root domain, then just add the root domain as one of your app's domains. dokku domains:add myapp mydomain.com.
That was really straightforward, the docs need to be updated to reflect this, really.
For your second question, your app is not visible to the outside world. Your app is running inside its own docker container, with its own local IP address. If you still want to find out what port your app has exposed, you can run docker ps on your server.