First, make sure you have installed the game into Steam, and then download the 18+ DLC from your account.
1. Go to Steam, right click on Ne No Kami. Go to Manage -> Browse Local Files
2. Click on Local Files. The folder should come up.
3. Open the downloaded file. We recommend latest 7zip for this:
(Make sure you open the en file for the English patch!)
4. Copy the contents over to the install folder (the folder opened in step 1)
5. To test, open the game from Steam. The title line should look like - make sure the [append] is there.