Multi Function Gauges

Hi all ,new to forum and looking for some help ,I'm in Ireland and have a new boat with twin DF90As connected to two separate multi function gauges ,on these there is an option to set the amount of fuel ,I have tried everything and cannot get the gauges to do anything with fuel, is there any step by step on how to do this or has anyone any experience in doing this ,the only thing it tells me is the amount of fuel used and I don't know if its for one or two engines ,I would like also to connect this to my garmin 7412 ,the engines are connected by NMEA 2000 set up . is it as easy as using the NMEA 2000 lead which came with the garmin unit and connecting it to the engine set up ,regards ,dezzer...