Please follow the screenshots below to sign into the Google+ Leaderboards / Achievements. Please note that you will not have access to your Local High Scores and Achievements while you are signed in. To access your Local High Scores and Achievements, you need to sign out of Google+.
(The second-to-the-last step chooses to access the Leaderboards. If you'd rather see your Achievements, click the button above it instead.)