I'd add it before fermentation and after fermentation to either backsweeten or carb. Maple syrup has solids that don't ferment out which give it more of a taste. Be prepared for a long ferment time, my maple cider has been fermenting for over a month now.
If you're interested, search the forums for the terms 'cyser' - an apple cider with honey or an 'acerglyn' - a type of mead made with maple syrup which might give you some more info.
Primary: Cherry Melomel
Bottled: JAOM, Amber Ale