To have a easier time getting the achievements in this game: Pause the game, go to Game Settings, select 5 lives and activate Round Select, you will get the achievements with ease this way
The Keys appear below the Ghost House after eating a certain amount of Pac-Dots, when the Key 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 13. You'll get the achievement once you eat it.
The game gets a bit tougher here, so i recommend going for the Key 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. Something that can help is getting eating a Power Pellet and immeadiately going for the Bell or Fruit, that way, the ghosts won't kill you if they get near you, but it's best to leave the Power Pellets to get more time to eat the Pac-Dots rather than the fruit.