Run the application from your android phone, you should see login page.
Configure your DNN portal url from Settings page from the menu:
Select "Web Service URL":
Type in your DNN portal address in the text box:
Login to your inbox using your username and password, you should be able to see list of your messages.
Click one of the message to see the content of your message:
When new message arrive, you will see a "New message" notification along with the notification sound, and your new message should be in your inbox automatically.