Bam Margera!
To unlock Bam as a playable Jackass at the main menu hold R1 and L1 then press Circle, Square, X then go to a stunt then select your Jackass, Bam should be there. (You can also complete the MTV Story Mode at 100% Completion to unlock Bam).

All Episodes
L2,L2, Circle, Circle, Square, Circle, Triangle, Circle, L1, Circle, L2, Circle.

All Jackass
L1, Circle, Left, Left, Circle, L2, L2, Right.