Big hat tip to Mike Arnesen of SwellPath for inspiring Paul Shapiro to turn Mike's code into a Wordpress plugin and in this video you will learn how to install the plugin on your Wordpress site and make your first Google+ Interactive Post.
The video to set up the Google+ API access is at - https://www.youtube.com/watch?v=w2q8K2-ajhY
You can read Mike's initial post on setting things up "manually" at http://www.swellpath.com/2013/04/google-plus-interactive-posts/ which might be handy if you are not using Wordpress.
Paul's blog where he explains the Wordpress configuration is http://blog.paulnshapiro.com/new-google-plus-interactive/
Finally, to get hold of the Wordpress plugin you can get it from Paul's own interactive post on Google+ at https://plus.google.com/112810035141633619131/posts/8eUh6VEa24D