Instructions
Go to the main menu in-game and click on the Shopping Cart icon to enter the in-game store. Alternatively, go to the song selection menu and select the "+" symbol.
Click on the music symbol icon to bring up the songs available to download.
Click on the track you want. Select the Shopping Cart icon to buy the track.
Select the Shopping Cart icon to buy the track using Nintendo Points via Nintendo's "Pay & Play" option.
Wait for the track to finish downloading and go to the track listings page to play the song.