To use the API, you must first configure your game in the Google Play Developer Console. Follow the instructions on creating a client ID. Be particularly careful when entering your package name and your certificate fingerprints, since mistakes on those screens can be difficult to recover from.
If you intend to use real-time or turn-based multiplayer in your game, remember to activate those features in the Google Play Developer Console when creating your application instances.
Copy the game resources from the console
Once you configure at least one resource (event, achievement, or leaderboard), copy the resource configuration from the Google Play Developer Console, and paste it into the setup configuration in Unity. To get the resources go to the Achievements tab, then click on "Get resources" on the bottom of the list.
Select all the contents of the resources window, and copy them to the clipboard.
Paste the game resources into the plugin setup dialog