1. Go to Steam, and right click on 9-nine: Episode 1. Click on Properties, then Local Files. Click on Browse Local Files.
2. You should see something like:
3. Download the DLC. In either 7zip or WinRAR, open it, and you will see something like:
4. Per the readme, copy these files into the folder you opened up containing the game files. It should look like this:
5. It's installed!