You must purchase a song before you can license it. All songs you have purchased are listed if you go to "Tracks" and filter for purchased tracks by using the checkbox "Only show purchased tracks" in the right sidebar. If you want to know how to purchase a track, see "How do I purchase tracks?".
To license your purchased track, go to "Your songs" and first register it by choosing the song you want to license in the drop down list, which license you need (depending on how many seconds you want to sample) and a description of what elements you want to sample.
Once registered, go to "Your songs" and click license on your registered song.