Maltodextrin is not a fermentable sugar. It does however, add better mouthfeel. So if you use 500g maltodextrin and 500g of dextrose with your kit it will pretty much produce a mid strength beer. The kit tastes great as it is, but if you want a bit more spicyness and zing, steep the hops in 500ml of boiling water for 10mins then add to your wort.