When you click on a mixtape, you should see a download icon at the bottom left of the screen. Once you tap on that icon, you'll be able to select what songs from the mixtape you want to download. Then tap on the Download button at the top right of the screen.
To download a single, tap on the single, then tap on the same download icon at the bottom left on the screen. That should download the song to your Spinrilla Library.