Sugar can be added during the boil or during the fermentation. It depends on what kind of mouthfeel you want. During the boil will make it easy to incorporate, but the problem is that the yeast will eat all of the simple sugar during the ferment and may leave some of the maltose behind, leaving a higher FG than expected; a fuller, sweeter beer. If you have regular table sugar, you can instead add it to the fermenter as it starts to slow down. This way the yeast will have its fill of maltose, then work on the simple sugars, creating a dryer beer. As far as I understand it table sugar is the same as candi sugar and there is really no benefit to it. If you have rock sugar already, I'd just add it to the boil within the last 15 minutes or so. I'm not sure how well it will disolve in the fermenter. Hope this helps.
