To Open every Player Cartridge you need to collect a item in their chapter.

Player 1

Is open by default.

Player 3

Go to the camera room in The Basement chapter and look at shelf in your right.

Player 2

On top of a white box  in the Beyond the World chapter. After climbing a ladder, keep going to your left to find it.

Player 4

On the left table in entrance of the loop corridor in The Loop chapter. You need number 14 on the right table to see it. It's right after defeating three monsters.

Player 5

On the third floor of the bookshelf in the living room after opening the door with the giant key in the Mini Mita chapter.

Player 7

In the Reading Books, Destroying Glitches chapter, after Mila stops talking go to exit door, open it and then close it, now you are able to go anywhere in the house. Go to bathroom and look at the shelves in your right.

Player 9

Go to Creepy Mita's bedroom in the Old Version chapter and look at the box in left of the computer.

Player 8

After running from Creepy Mita in the Old Version chapter go inside the Core room and look at your left to find in on a chair.

Player 10

In the beginning of the Real World? chapter go the bedroom first and look at the table in right of the bed.

Player 6

In the Reboot chapter, after getting thrown by Core Mita, Mita will ask you to check the basement. Go to the basement and check the teleport device next to the cell.