Preheat oven to 350 degrees. Pour the melted butter in a 9x13 pan. Sprinkle brown sugar evenly over melted butter. Arrange pineapple slices on top of brown sugar. Place cherry in center of each pineapple slice, and arrange remaining cherries around slices; press gently into brown sugar.
You should have 1 cup's worth of pineapple juice reserved. If your can of pineapple didn't quite give you 1 cup of juice, then add enough water to the juice to make one cup. Make cake batter as directed on box, substituting pineapple juice mixture for the water. Pour batter over pineapple and cherries.
Bake 45 minutes or until toothpick inserted in center comes out clean. Immediately run knife around side of pan to loosen cake. Place heatproof serving plate upside down onto pan; turn plate and pan over. Leave pan over cake 5 minutes so brown sugar topping can drizzle over cake; remove pan. Cool 20 minutes. Serve warm or cool. Store covered in refrigerator.