You go to the book icon and press 'POINT.' It will tell you how many points you have. Then yo press the 'B' button and it will ask: 'donation for the king?(yes/no)' and you press 'yes'. Then you punch in how many points you want to donate and your done!
Also, if you give 5000+ points, you will be eligible to visit the king (make sure you write down the password to visit him, it will be on you tamagotchi!).