The Melons appear below the Ghost House after eating a certain amount of Pac-Dots, when the Melon shows up, stop everything you're doing and eat it, or else, it will disappear and you'll have to either reset the game or try again on Round 8. You'll get the achievement once you eat it.
This is where the game gets a bit tougher, so i recommend going for the fruits while the ghosts are far away from you, otherwise, you'll lose a life or get a game over depending on how many lives you have.