Loading a Custom Story(MAC)

[Corvo]

So I just finished SOMA and I wanted to tryout the [Developer Test]Small Custom Campaign, so I downloaded it on steam, but I cant find how to play it. I've tried finding the "second play button" and modlauncher, but I cant find it, I even created an folder called "mods" on steam>steamapps>common>SOMA>mods and put the downloaded content there.