I found these cheats for Smurfs' Nightmare on CheatPlanet.com.
First put the Smurf with the face that has the glasses in the first space.
Next, put the Smurf that has the pencil behind his ear in the middle space.
Finally, put the Smurf that has his mouth wide open in the last space.
When you do this it leads you to a course where you ride a rabbit to find the key to the next door.
The Strange Planet
To get to the Planet level, put the Smurf with the bubble around his head, the Smurf that looks scared and Brainy Smurf.