1. Make sure you have Source SDK Base 2013 Singleplayer installed (Half Life 2 is NOT required). To install it, go to the "Library" window on Steam, select "Tools", and install "Source SDK Base 2013 Singleplayer".
2. Extract the rar file into the following folder /../steam/steamapps/sourcemods
3. Restart Steam. You should see "Hopeless Night" in your game library.