Trouble Updating Game Help pls

[SpicyDragon]

I have the disc version of Amnesia The Dark Descent and I tried to install the 1.3.1 update and when I went into the game files I saw these things called Vcredist and I have no clue what they are or do.

I also launched the game and it works fine but I don't know how to activate the Justine DLC, and when I try to launch the game from the actual files it says an SDL.dll is missing? I don't know what that means either.