Instructions
Launch "Mario Kart DS" and select "Single Player" from the main menu.
Select "Grand Prix" followed by "100cc."
Choose a character and a kart.
Opt for one of the "Nitro Grind Prix" cups: mushroom, flower, star or crown. Each cup has four races you must complete.
Place first overall by the end of the four races to receive the gold trophy.
Receive the gold trophy on each cup in the 100cc Nitro Grand Prix to unlock the third kart for each character. Your progress is saved after each cup so you do not have to complete them all in one sitting.
Select Yoshi from the character selection screen. Tap the left or right arrow on the kart selection screen to change your kart to the cucumber kart.