It's just like earning TamaTomos on the iD: you have to feed or give your Tamagotchi certain items to play with to make your Tamagotchi happy. Each character responds to certain items. You may need THIS topic over at Tama Zone.
You find the happy items for your tamagotchi. Go to google and search 'tamagotchi id l happy symbols' for your tamagotchi's happy items. The last happy symbol is letting your tamagotchi character be an adult for 48 hours. Once you get all four of them, your tamagotchi will count as a tama tomo character. It will be added to the book, and every four tamagotchis that reach tama tomo level, you will get a destination in your door icon!!! Good luck.