Don't click in the box, just type in your age, then click on the scroll bars for the height and width. "I mean weight". Other wise this won't work properly.
Enter your age. Use the slidebar to enter you height. Your optimal BMI will be calculated. Move the slider for your weigth. Your BMI corresponding to your height will be displayed. An informatinal self explaining text will be displayed. If your BMI is OK the background will change to green.