After unlocking Tails, select "No Save" mode and then the plus for both Sonic and Tails. Finally, tap the letters "A", "G", "E", and "S" in order at the at the Sega screen to change Tails' name to Miles. A Japanese message will confirm correct code entry.
After unlocking Tails, select "New Game" and then the plus for both Sonic and Tails. Hold UP and press JUMP during game play, then hold UP and press JUMP again while in mid-air to start flying with Tails. Press JUMP at Tails to hang on, then use the D-pad and JUMP button to fly.