In microwave-safe bowl, microwave butter on HIGH until melted, about 1 minute. Stir in sugar, corn syrup, pineapple juice concentrate and rum extract, if desired. Microwave on HIGH until mixture boils, about 3 minutes, stirring once. Microwave on HIGH 3 to 4 minutes without stirring. In large, shallow microwave-safe baking dish, combine popped popcorn and coconut. Slowly pour in juice mixture, stirring to mix. Place in microwave oven and microwave on 75% POWER approximately 6 minutes, stirring every 2 minutes. Turn popcorn onto wax paper or aluminum foil and allow to cool completely. Break apart and store in tightly-covered container.
Yield: about 3-1/2 quarts.