I hate to be the one to ruin it, but I can't resist :P. The secret achievement is achieved by clicking the little marshmallow that walks across the main menu.
oh, I went to your profile and I can see this is your first game. It's good for a first, and you should keep on making games. The best thing you have going for you is that there are no glitches (that I have found). Just keep making games that you enjoy making, and you will do just fine :).
 "Transformice"){kind=icon}
 "Feudalism"){kind=icon}
 "Dead Drunk 1.3"){kind=icon}
 "Marshmallow Kingdom"){kind=icon}
 "Gravitee 2"){kind=icon}
 "Road of the Dead"){kind=icon}
 "Dodge the Squares"){kind=icon}
 "Ray Ardent: Science Ninja"){kind=icon}
 "Doodle Devil"){kind=icon}
 "Fast & Furious"){kind=icon}
 "You are a... The Sequel"){kind=icon}
 "Fly an Airplane!"){kind=icon}
 "Zombies Took My Daughter!"){kind=icon}
 "The Undead Survival Test"){kind=icon}
 "Gravitee Wars"){kind=icon}