The most convenient option to eat while you are at ISSP is to dine at campus cafeteria. They are open for lunch and dinner during weekdays, and for lunch on Saturdays.
To have a breakfast while you are staying on campus is bit tricky. The campus guesthouse unfortunately does not provide breakfasts. The easiest option is to buy something at a convenience store nearby. The closest one is the "Family Mart" which is indicated on the information map. A better option might be bakeries in the neighborhood; they are open from 6:30am. Again, you can find their location on the information map.