First of all remove the HTML in the index.php, most times it isn't needed.
Well the plugin is a normal plugin. You'll need to understand how a plugin works first. A plugin has a trigger text { popin }. When you put the trigger text in an article, this gets replaced by the plugin functionality (in our case, the popin window). So make sure the popin plugin is enabled, and put the popin text in an article. This should get replaced by the HTML in the popin parameters. The trigger text does not show here, because it gets replaced with the popin window
