All you have to do is hold down 1 and 2 on the title screen and keep holding it down until you are on/in the observatory. Then you will hear a strange noise. Go find the mail toad (he is usually in front of you when you start) who will have a speech bubble above him with an envelope in it. Talk to him and he will give you a letter from Peach. After you have finished reading it he will say that Peach attached 5 1-up mushrooms and he will ask you if you want them. Always say "yes" otherwise they will be gone forever.