The pendulum is basically an amplifier. It is a tool that helps you to access the collective unconscious and your Higher Self. You can get answers to anything.
Every one is basically capable of working with the pendulum. Learning to work with it is like learning to play an instrument. It requires constant practice and experimentation. Making mistakes is part of the learning process. The results you get will reflect your stage of spiritual/mental development.
You should not use the pendulum for egoistic reasons. Use it only for the well being of others or yourself. Swinging the pendulum requires that you be undivided and totally present in the moment.