Add water to jelly powder in medium microwaveable bowl. Microwave on HIGH 1-1/2 min.; stir 2 min. until jelly powder is completely dissolved. Add marshmallows; stir until evenly coated. Microwave 1 min. or until marshmallows are puffed; stir until marshmallows are completely melted and mixture is well blended. Pour into prepared pan.