Stage Select:

In the title screen, while "Press Start" is blinking, press B, A, Right, A, C, Up, Down, A (just as if you type "BARACUDA" with the buttons and the first letters of the directions). You'll hear a sound if you've done it right. Start the game and the Stage Select will appear. You will be able to select any Tails or Knuckles 3D bonus stage, too.

 

Skip Level:

If you entered the Stage Select code, to skip a level you can pause the game and press A.