This Achievement is for finding the secret area on Mission 01 Stage 01 : The City.
Once you start the stage move forward until you get the Ninja Power up.
Now backtrack to the start of the stage. Double Jump up to land on a hidden platform.
Double Jump again top land on another hidden platform.
A platform will come down , jump on to this to be transported to the Hidden area which contains 2 extra lives (2 x 1UP)