Change any value and click "Calculate" and the IBUs will be calculated. If you want to make multiple additions, simply fill in another line's Alpha Acid Percentage, Mass Added, and Time Boiled fields. The Utilization Percentage, Bittering Units, and Total Bittering Units fields are calculated for you; any entries you make to those fields are ignored.
The Bitterness Calculator takes into account both the time of boil and the wort gravity during the boil. The numbers are from Glenn Tinseth, publisher of The Hop Page, based partly on data and partly on experience. Your experience and brewing practices may be different than his so here's the official disclaimer: Your Mileage May Vary (YMMV).