Achievements and leaderboards are only available on the following stores:
- Google Play
- Amazon Appstore
- iTunes Store
When starting the game, you should see an overlay appear for the respective platform prompting you to log in - tapping it will let you access these features.
You can also find an in-game menu option for each depending on which store you bought the game from. If you got the game from another store, these features are not enabled.