Beat all the levels of party cruise and defeat Bowser. Bowser's World will be unlocked in deluxe and party modes.
First you need 1000 cruise mileage points. When thats done, go to the duty-free shop. Hover the hand over the computer and hit A. Hit A again on the shadow of a thing with a bow. It will ask you if you want to trade your mileage points for Birdo. Hit yes and Birdo is now a playable character..
Buy it in duty free shop with 1000 milleage points
Thanks to Revolution readers DynamoDom and pppartyppants!