How to Add Media to a Playlist from the Media Editor
STEP 1: From within the Media Editor, select the Playlist tab at the top right.
STEP 2: At the bottom on the Playlist tab, select the 
button. This will reveal all existing Playlists within your account.
STEP 3: Select the Playlist you wish to add your Media too, then select Add at the bottom right of the screen. Make sure to click on Save when you are done.