Donkey Kong's FinaL Smash is called Konga Beat. He takes out his DK Bongos and starts playing to the beat.
You have to press buttons to the beat. The more you feel the rhythm, the more power you will have.
And that's the end of his attack. DK is invulnerable during his Final Smash, so just relax and enjoy the music. Sakurai also says that jamming during screen scrolls is something we might want to think about. What he means by that, I don't know.