With Americans fattening up and fast food on the defensive, McDonald’s Corp. this week began telling dieters in the New York area how much fat and carbohydrates are in some of its meals.New posters and brochures, prominently displayed in restaurants in New York, New Jersey and parts of Connecticut, tell customers how to modify McDonald’s existing menu--by leaving out the bun or cheese, for example--in order to reduce their intake of fat, carbs and calories.