To get this achievement is very simple:

1- In main menu screen(not levels one) press the following controls:
Up,up,down,down,left,right,left,right,x and z

If you hear a sound,the cheat was activated with success.

Attention:
If you changed the default controls,press the correspondent buttons below:
x is the run button
z is the jump button

Hope you all like this very simple guide!