That documentation is out of date. You don't have the Google+ API enabled. Navigate to
console.developers.google.com
then at the top click the dropdown and select your project. Next click Library and select Google+ API under Google+. At the top should have a toggle to enable it. You can then configure the necessary oauth credentials on the Credentials page for your Client ID and Client Secret.
A new tutorial will eventually be made, but I honestly can't keep up with these social sites changing the UX every other week.