At the pause menu, enter the following:
Up, Up, Down, Down, Left, Right, Left, Right, L1, R1, L1, R1
Finish the game than go to the extras section and beat the 5 dragon challenges
You will now be able to use Dark Spyro in a new game by pressing L3.
During gameplay pause and press up, up, down, down, left, right, left, right, L1, L1, R1,R1