Thanks, Mateja. I am not using WordPress.com but WP installed on a self-hosted site (v3.8) and I cannot find the “related post” feature described on your link. It seems as it is a WP.com’s blogs feature only…
Yes, I assumed that you have a self hosted WordPress and yes, feature on the link is integrated into wordpress.com.
Well, if you’re looking for a related posts plugin, I’d suggest you search for related posts on WordPress plugin directory. There are plenty of good plugins there that can be configured to look similar to the related posts on the demo post example.
You can even try our Related posts plugin. But there’s no pressure 🙂