Be careful if you use lactose. While it is excellent in stouts, it is easy to have bottle gushers from infections. While yeast cannot generally digest lactose, it can get lactobacillis in it which can digest lactose and raise the bottle pressure substantially. This happened at Lancaster Brewing in PA a few years ago and they had to recall a lot of beer. I have used a wine making additive enzyme called lysozyme which is a natural enzyme in human tear ducts, sinus drainage and saliva and can kill gram positive bacteria.
We have not used lactose, only honey, brown sugar and table sugar. And the Mr. Beer kits are hard to mess up. :-)