While in a park, press X, B, Y, X, B, Y, B. If you entered the code correctly, 
you will hear a sound. Press X at the mission selection screen to complete the 
highlighted mission and unlock the next mission. Note: Each time X is pressed on 
a highlighted mission, the rank of the completion increases (from Bronze, to 
Silver, and finally Gold). To reverse your progression on a mission, press B at 
the mission selection screen to remove the highlighted mission medal.