I wouldn't add fruit in the primary just because there's a ton of sediment already at the bottom and you don't want to have to work around more.
As for how much to add, it all depends on how strong you want the flavor. For a strong flavor, I usually add a pound per gallon. But, with an american style light beer, you won't need much to get a noticeable flavor. Maybe 2-3 pounds total. I add it to the secondary and I usually use fresh fruit that's been frozen, or Oregon Fruit Puree.
Don't use the flavorings you find at your LHBS...most of them suck. Cherry will leave your beer tasting like cough syrup. The only ones I've found that are good are raspberry and peach.