Ufotchi is not an adult, it's a teen. You use the love potion by making your Tamagotchi drink it when it's a adult at the age of 5 or more. Then you connect with a Tama of the opposite gender and your Tama will gain all the hearts necesary to mate with them.
The screen will go black, you will see fireworks, and when the screen clears, you'll see 2 tamababies next to the adult. One will go over to the other tamas screen.