Assuming you have a MusicStar (V6) Tamagotchi, and are trying to log into MusicCity:
If you haven't already done so, you need to create an account on MusicCity. This will give you a MusicCity Account User ID and User Password.
When you want to make your MusicStar character go into MusicCity...
First: you log into MusicCity using your MusicCity Account User ID and User Password. At this point a guest Tamagotchi character will appear in MusicCity.
Then: you log your MusicStar character into MusicCity using your MusicStar User Name and Login Password (both of these come from the MusicStar toy.) Now the guest Tamagotchi character in MusicCity will transform into your MusicStar character.