This video shows you how to set up a Google+ service on your account. You can find Services at the top of your dashboard, click Add. If you're already signed in to Google+ you will be directed to sign in otherwise all that is left is accepting permissions.
After that you reach the Services page, here you select the page you would like to add and where you want to receive alerts for new activity.
Save Settings and start enjoying your social media management of Google+ using the Compose Message box.