Your Tamagotchi has to be an Adult in order to use the costume. Scroll down the item menue untill you see the item you want to use, then press the "B" button. If you want to take it off, then set the tama's time to its bed-time and it should be off!
WARNING: if you take it off, you can NOT re-use it, you whould have to buy the costume again. I suggest you keep it on until you don't want to use it anymore.