First, make sure you have installed into Steam, and then download the 18+ DLC from your account.
1. Go to Steam, right click on Ne No Kami, select properties, and then go to Local Files. The popup should look like this:
2. Click on Local Files. The folder should come up. Then open the zip and move the file into it. The result should look like:
3. To test, open the game from Steam. The title line should look like - make sure the [append] is there.