The D2 comes with a files app. Use it to go to the location of the file, press and hold, menu comes up, move or cut, then go to the location you want it in, paste.
You can use the music app to find the song long press on it and it will ask if you want to make it the ringtone. From that point it will stay in the list.
You can also choose it from the music source instead of system source when trying to make it a ringtone.