Instructions
Select Yoshi Park on the main map screen. As soon as you enter the park, you will be approached by two Shy Guys who will ask for your help. Help them by using the "A" button on the Wii remote to fire baseballs at the piranha plants, which are located on a Ferris wheel. After destroying the piranha plants, a warp pipe will be revealed near the merry-go-round where one of the plants was located.
Jump into the warp pipe, using Mario, and follow the path to a manhole cover. A Red Magikoopa will appear and give you a challenge: catch fly balls. Catch three balls before they hit the ground to finish this challenge. Aim your character for the target ring that appears and wait for him to catch the ball. After beating this challenge, Yoshi will jump out from beneath the manhole cover and join your team.
Switch to Yoshi. Use his ground pound power on the blue manhole cover. You will pop up in a new area, where Blue Yoshi can be found. Talk to him; he will ask you a series of questions. Enter the correct answers into a keypad near a roller coaster and Blue Yoshi will join your team.