Once you have registered for a free MultiTracks.com account you are able to make purchases and have that content be readily available for you in Playback.
In order to add a song that has been purchased, rented or uploaded (using MultiTracks Cloud) you will need to tap on the "Add a song" button. If you already have a song loaded in the playlist you can tap the "Edit" button and then tap the small "+" symbol that appears on each song tile located in the playlist. This will then give you the option to add a new form of content into your playlist.
If you run into any issues adding songs, content or Rentals into Playback please submit a request and be sure to select the Playback category from the drop-down menu.