Press X, B, Y, X, B, Y, B when you are in a park and if you enter the code correctly you will hear a confirmation noise. When you have heard the confirmation sound press X at the 'Mission Selection' screen to complete the highlighted mission and unlock the next mission. Every time X is pressed on a highlighted mission the rank of the completion will increase from Bronze, to Silver, and finally Gold. Press B at the 'Mission Selection' screen if you want to reverse your progression on a mission and remove the highlighted mission medal.
Added by: Sanzano Aug 18th 2009, ID#4853