If you haven't finished the game(on any story) then go to Tails's workshop.(I don't know
if you have to go in or not.)If you have finished the game,(on any story)then go to the
harbor at Centrol City or That one place under the train station and where you have to
climb down a ladder to get down there in the Mystic ruins.There should be a bout at both
of those places.Get on and you'll be there.