Double click to enter the playlist and click 'Add Content' on the top right menu. This will bring up the media library pop up and allow you to select the content you would like to add. You are also able to add new content into your media library by clicking 'browse files' or dragging content into the upload box.
Within the pop up media library you can search or navigate to a different folder. Once the media is added into the playlist you can rearrange the order by dragging them to desired spots, change the individual media durations and duplicate and delete content.
When you have finished building your playlist click publish to send your playlist to your screen(s). In this pop-up you can select the screens you'd like to assign this playlist to.