In microwaveable measuring cup, microwave butter uncovered on High about 30 seconds or until melted. Stir in brown sugar and pineapple juice concentrate. Microwave uncovered on High about 30 seconds or until hot; stir. Pour over cereal mixture, stirring until evenly coated. Microwave uncovered on High 6 to 8 minutes, stirring every 2 minutes, until mixture just begins to brown.