Wolfson is right. Using a defib during asystole (completely stopped) is mostly useless. I wouldn't rule out the possibility that it could help, but it's supposed to be used during ventricular fibrillation (hence "defibrillator"). It can also be used in some other rythms. From my limited understanding, many people go into ventricular fibrillation before asystole. It is during this time that they are "pulseless" but the heart is still reacting to electrical signals.
Most defibrillators these days are automated. You turn the thing on and it gives you instructions on how to place the pads on the persons chest. From there it takes over and decides whether or not to shock, and whether or not to administor CPR.
Typically, if the person is in ventricular fibrillation (or another shockable rythm), the unit will shock once using 200 joules. If no change in rythm is detected, it will up the shock to 300 joules. If there still is no change in rythm, it will up to 360 joules [the numbers vary depending on the machine]. If all three fail, it will tell you to begin CPR for one minute. After a minute, it will try another three shocks. After 6 shocks, the EMT's and Paramedics will continue CPR until they reach the hospital, where the person can be pronounced dead.