So ...

There are two different ways that will allow you to display the real-time online users: you can create a standalone page or you can add the online users widget to your sidebar (or any other widget-ready area).

Let's see the first method ...

Upon activation, go to Settings => UserOnline to access the plugin configuration page. This is where you can customize the display settings, the time out, the users online page URL, etc. The default settings will work for most sites:

What is important: when you create the new users online page, the page’s slug must be the same as you had in the plugin’s settings. Then, simply add the [page_useronline] shortcode into the content area, and publish the page:

And here's the live result:

OK. Now let's see the sidebar widget ...