BEFORE you integrate Google Leaderboard into your game, you need to setup GOOGLE PLAY SERVICES. Please follow this guide.
Done? Let's continue...
Open GAME SERVICES tab & SELECT YOUR GAME
LEADERBOARDS => ADD LEADERBOARD
INSERT LEADERBOARD NAME & PRESS SAVE AS DRAFT
PUBLISH THE CHANGES
and press PUBLISH CHANGES
REMEMBER LEADERBOARD ID
COPY / PASTE leaderboard ID and App ID into your game in QuickAppNinja wizard
Now you need to TURN ON LEADERBOARDS, GENERATE NEW GAME VERSION & PUBLISH IT TO GOOGLE PLAY
Now you will see new option in the game menu