To find any available extra content that comes with your game, right click the game from your stash page after downloading (In the RC client) and click on “Open Installation Folder”.
You’ll be redirected to your game’s installation folder. Click on the “EXTRAS” folder. (Note: Some extra content may be compressed and/or in an archive format, simply extract and/or uncompress these files.)
You can now access all your game’s extra content! Take note that these extra files will be deleted from your library if you choose to uninstall your game through the RC client. Don’t worry, they will re-appear again when you choose to download and install the game/s again.