Try setting aperture to a low value, say, F3 to F5. This will let more light into the diaphragm. Set shutter speed to something fast, like 1/100 or 1/200. set ISO to a high value. Your SLR should be on the "M" part of the knob, which stands for manual.
If all else fails, use "full auto mode" or "portrait mode" until you get warned by security
