Instructions
Save a "Super Mario Galaxy" file on your Wii. Race 50 races on Mario Kart. You will then see a message stating that "Super Mario Galaxy" gave you a present. Rosalina will be unlocked on your game.
Start "Mario Kart Wii" and choose "Grand Prix" mode if you do not have "Super Mario Galaxy." Choose any character and any kart.
Win every race in every cup and in every mode. You have to get first place on every race track in the 50cc division, the 100cc division and the 150cc division, a total of 96 races. This will unlock "Mirror Mode."
Obtain a "One Star" rank or better in all Mirror Mode cups. You will then unlock Rosalina.