its easy to use - firstly, connect to the top port of the merlin, Once you have that done make sure the valve on top of the flushing kit is open, turn on water supply, make a tank of water, switch off water supply and thats it - it will flush the merlin automatically.
if your water pressure is low you may need to release some of the pressure from the flushing kit before connecting it up. run it for a time and while the tank is filling lift the flushing tank to see if there is any water in it - if not let some pressure out - if there is then leave it alone.