Paste the downloaded files (steam_api64.dll & steam_api64_o.dll) into the games root folder (\SteamLibrary\steamapps\common\TheEvilWithin)

Boot up the game, and you should receive a popup with the headline "The Fighting Chance Pack"

In case you don't receive the popup, delete all files in your save folder and reboot the game. C:\Users\YOUR_USERNAME\Saved Games\TangoGameworks\The Evil Within\base\savegame