Press Up, X, Down, B, L, Y, R, A during the introduction when Vegeta fades into Gokuu. A sound will confirm correct code entry. Adult Trunks will be above Majiin Buu on the character selection screen.
Turbo Mode
Hold A + B + X + Y + L + R on controller two and power on the SNES system. Release the button after Goku's Kamekameha is introduced. The "3" in the title will turn orange to confirm correct code entry.
Adjust Power Level
Slowly enter one of the following codes before a match, while your character is speaking to the opponent.
1 - Up, L, Down, Y.
2 - Left, L, R, Right.
3 - Down, X, Up, R.
4 - L, R, Up, Down,Y, X, Left, Right.
5 - Up, L, Up, R, Up, Y, Up, X.
6 - Left, R, Up, X, Right, Y, Down, L.