How can I toggle Dropbox "on" to Import from the Files app?

You can use the "Files" app selection to import and export from Dropbox if you have the Dropbox app installed on your device.
To do this:  Open OnSong > Songs > + > Import section > Files > Select the Browse tab from bottom > Tap edit above the locations column > Toggle Dropbox to On > Tap Done > Select Dropbox > Find the folder and song you want to import > Tap on the song you want to download/import.