Not knowing this particular flashgun, but....

On the back of the unit (on the diagram/graph/gauge), there should be two coloured lines (one yellow, one green). These lines should extend from one end of the scale to a specific distance & f-stop number for each ISO setting (e.g. 2.8 & f4 @ ISO 100). To use the A settings, select one most appropriate one via the switch on the back & set the lens' aperture to this value. As long as the flash-subject distance is less than the one where the coloured line stops, the exposure will be correct.