Choose your character in "ARCADE MODE".When you choose MANUAL or AUTO option for your character,Choose with the buttons A,B.Hold them until the fight starts.Instead of fighting the regular character,you'll fight AKUMA.
After selecting your character and Normal or Auto Mode, Hold down A, B, and Select until the match starts. Bison will come out and you can fight him. This code works no matter what stage you are on or who you will fight next. If you want to fight him in the middle of the game, Hold down the buttons after winning the final round.
To do this, do everything in this order
Begin a game and intentionally loose the match.
Choose "Continue" at the continue screen.
Defeat the character and loose to the next
Repeat steps 2 and 3 until you have been
defeated by all of the game's characters.
After losing to Bison, continue a new game as Adon.