This is earned by finding all 9 pieces of the disc and then pressing [cn_Y] at the game console in the arcade. You can begin as soon as you get 20 stars and enter the arcade for the first time. The girl there will give you a hint on their location each time.
The locations are as follows:
1. Right behind the girl in the arcade, in the upper right corner.
2. Just outside of the arcade, climb up the tall pole to the lookout spot - it's in the air between two of them.
3. Go to the generator room (to the right of the arcade, with Benji inside) and jump up and over the generator to enter a secret area on the right side.
4. Go to the Lobby, then go all the way to the left to get to the costume area. Continue left and jump into a slightly discolored area along the far left wall.
5. Go to the Lobby and jump onto the chandaliers, then move across them to the far upper left and jump into a discolored area.
6. Jump onto the top right level of the main area and go right behind the curtain. You'll need to do some platforming.
7. Jump onto the middle level of the main area and go left behind the curtain.
8. Once you unlock the engine room at 120 Stars, enter and jump over and behind the engine.
9. Once you beat the game (by entering the engine room for the first time), head to the far right of the ship (past the generator room) and jump off. You'll be in the tutorial area. Backtrack and you will get it in the final area where your house is. Enter your house to return to the ship.
Once you're back on the ship, simply head to the arcade and press [cn_Y] at the games console.