Put it in the DC mA mode. You may have to move the red lead plug to a different hole. Connect the meter leads in series with your lamp, polarity doesn't matter, and then connect one meter lead and the other lamp lead to the 12V power supply. The lamp should light, and meter will read the current required to operate the lamp. When you put the meter in current measurement mode, and hook it in series with the lamp, the voltage drop across the meter is so low that the lamp will not know the meter is there...