On those I believe on places the film's point of interest over the crosshairs on the little light table. Use the "DENSITY" button with the upward pointing arrow (far left button).
Place the opening on the base of the reading device over the negative point of interest and press the receptor downward. The density should appear on the LED readout.
Before taking a reading, place the reading head over the crosshairs on the light table and press the WHITE circular button on the reading head to zero (or zero to film base + fog).