Once the application is deployed on your phone, click on the "dokkaner" icon to launch it. 
When you start, Dokkaner will offer you to download the new data available. If you launch the app for the first time you will also have to download all the elements to display. This operation may take a few minutes. Please make sure you are connected to the Internet under good conditions.
Once the data is downloaded, the main Dokkaner interface of showing Dragon Ball Z - Dokkan Battle cards will be displayed. (see here)
It is recommended to create an account in order to access all the features of Dokkaner.