The melon will appear in Rounds 7 and 8 after 70 dots have been eaten. It will also appear after 170 dots have been eaten.
If you're selecting Round 7 or 8 from the start, remember that each dot is worth 10 points. You'll know that you're getting closer to the item appearing when your points get closer to hitting 700pts and 1700pts (assuming you've not eaten an energizer/power pellet or eaten a ghost at this point).
Eat it when it appears, and the achievement will pop.
Don't forget, you can set your lives to 5 in the settings before the game. You can also choose level select, where you can select any level that you have completed in a previous game.