I think the difficulty you're having is with the new Google Developers Console.
At the bottom of your screen you should see a link named "return to original console" which takes you back to the screen used in the guide you mentioned above.
The new console is quite different - "Simple API Access" is now called "Public API access" and works a bit differently as well
When setting up the Google+ API you need to add "Create New Key" then choose "Browser Key", go backm select the new API and open "Credentials" to find the Public API Access key
I'm updating my API guide here to show the new method (may take another 30 minutes or so) http://tech.graphicline.co.za/reviews/social-media-tabs-plugin/