You must purchase a song before you can license it. All songs you have purchased are listed under "Tracks": filter for purchased tracks by using the checkbox "Only show purchased tracks" in the right sidebar. Your purchased tracks have a "License" button. If you wish to license a song, click "License" and follow the instructions. Before you agree to the license, you must read it carefully as it contains information you will need as to what you can and cannot do with the licensed songs.