I have purchased the DLC pass and downloaded the first DLC, "The Depths". I downloaded the file provided (a .exe that is somehow only 4MB, but that's what's listed on the store page for it, so I won't question it for now). The .exe installed to the game's install directory, which is how the other pieces of DLC installed (the scarecrow sack and the upside-down teapot), which work fine.

The DLC does not seem to be working though. From what I've gathered from the Steam forums for the game, you access it through chapter select, with The Depths being under the first chapter. It doesn't appear for me at all. I have a completed save file.

I'd appreciate any help, thanks.

Happening to me too.
installed the DLC, and not seeing it under the chapter select. Also got a completed save file.

I also notice whenever I press [Launch] after installing the DLC it for some reason opens up the Computer Window (The window that displays your HDDs and SSDs).

Download and install the latest version of the main game first. It includes all the DLC. The installer itself is actually just an unlocker.

Make sure, that the DLC is enabled in the settings of the launcher. There is a symbol right next to the play button, that allowes you to configure the launch settings. Just tick the boxes of the DLC you want to be able to acess. Solved thje problem for me. Hope this will help XoX