To start using the d!nk app for iOS you will need to install the app first.
Go to the App Store, search for 'dink' and click on the 'Install'-button.
The app will be installed after entering your iTunes password. You will now find it on the home screen of your iPad or iPhone.
As a note: If you get any notifications regarding an untrusted app developer, please read this article.
After you installed the app, you can now login.