If you receive an error 400 - Error: redirect_uri_mismatch from Google Plus when attempting to login, the problem is due to the wrong address specified in the Google Application.
Double check the Authorized Redirect URIs field in the Google Application and ensure that the final trailing slash is present. For example you must enter http://www.yourdomain.com/ and not http://www.yourdomain.com
Furthermore, ensure that the Instant Facebook Login module is published and assigned to the home page of your Joomla website otherwise the Google login won't work.
As an alternative, you can change the following parameter of the component in order to match the exact redirect uri set in your Google application, being it with or without the trailing slash:
I receive a redirect uri mismatch error when trying to login with Google, why?