Play as Day Smith
At the "Press Start" screen, quickly press Triangle, Up, Down, Up, Down, Square. Alternatively, in OK City, go up to the ramp that you put in place with the fork-lift and into the secret garage. Grind on the black box, and it will open up into a vert vamp. Use the ramp to grind onto the bullhorns, and jump into the small room to unlock Day Smith.