Selecting the button Choose, you will have access to your contents coming from jwplayer.com.
You have just to click on the field Select video/ playlist... to see the latest single videos and playlists you've added on JW Player.
Scroll down and click one of the five playlists listed, the thumbnail and the video information will be shown and you'll be ready to publish the content.
Using a self hosted player
With a self hosted player you don't have access to your contents hosted on jwplayer.com so, how can you publish a playlist?
It's simple, just add more than one video in your post or page, and use a shortcode like suggested by the plugin itself.