Music Star cannot be debugged.
On other connections (preferably V2 and V3) you have to take apart the Tamagotchi and use a pencil to scribble over the little golden pattern on the chip which says "debug". Make sure it's fully covered in pencil lead. Then, put your Tamagotchi back together and it will be in debug mode and you will see some pretty weird characters you can choose from! To un-debug your Tamagotchi, use an eraser to rub out the pencil lead you put on the inner chip.
V4 and V4.5 have debug buttons but all they do is make the Tamagotchi go in super fast time, and once you debug it, you cannot make it go back to normal.