The oversized sign is the entrance sign at the start of the "Eight Pages" level. You should encounter a type of cabin with a few collectibles near it. The sign to earn the achievement should be hung on the wall.
Easy "Insta-Killed" achievement
Play the Genesis level under the Hardcore difficulty setting. Run to find the first page, then find the other pages while looking for Slenderman. Once you see him, run to him to die. Collecting more pages while waiting for him will improve the chances of his appearance.
Easy "Master Pianist" achievement
Use the piano found in the house during the Prologue a few times until the achievement is earned.
Easy "Secret Slide" achievement
During the prologue, go to the slide behind Kate's house. Climb it's steps to reach the wooden fort, then step onto the slide.
Easy "Tangled" achievement
Reach the burning forest part of the "Arrival" level. Stop moving and allow Slenderman's tentacles to grow and get you.
Secret level
Play "Part 1: Prologue" and examine the swing set located to the left of the house. After getting the message, enter the house and play the piano until another message appears. Go to the back room behind the garage and turn on the radio. Take the flashlight, then walk outside and examine the post across the street to find the missing person flyer. Once you approach the flyer, you should die. Note: This may require a few attempts. Once at the main menu again, select "New Game" and the secret "glitched" level will start. The message "I'm lost. Can you find me?" will appear to confirm that everything was done correctly. You will now be able to explore the house. Note: There are three versions of this level.
Various Achievements
Complete the following achievements to unlock Xbox Live Gamerscore points.
Achievement
How to unlock
Cry Baby (10 points)
Disrupt Charlie while he's crying at least 3 times.
Fearless (40 points)
Beat the game on 'Hardcore' difficulty.
Fire Proof (30 points)
Make it through the forest fire without catching on fire once.
I Love Radio (10 points)
Listen to all of the radio tracks in the house.
I'm Free! (10 points)
Beat the game on normal difficulty.
Master Pianist (10 points)
Play the piano until the first notes from 'No Friends' is played.
