To enter the code hold down all buttons as shown. As well as pushing the analog sticks down, you will also need to move them in the direction shown in (). Enter the Big Head code whilst paused, and the Everything code on the main menu. You will here a noise if done correctly

Big Head (ATVs and Bikes)
Pause the game, then hold L1+L2+R1+R2+R3(whilst pushed right)+L3(whilst pushed left)

Unlock Everything
At main menu hold L1+L2+R1+R2+R3(whilst pushed up)+L3(whilst pushed down)