1. Go to Steam, and right click on Wagamama High Spec. Then right click on Manage, and then click Browse Local Files.

2. You should see something like this :

3. Go to where you downloaded the DLC file from Denpasoft. Usually, this is your Downloads folder. Then, open the file labeled 'steamextradlc.rar' in 7zip. 

Note: If this fails to open, update to latest 7zip from the website. 

This should look like this:

4. Open this up, and extract it to a new folder of your choosing. Make sure to extract the entire thing, and you should see something like this once extracted:

5. Run 'Wagamama HighSpec_Steam_R18DLC.exe'.  You should see this when it starts:

Note: If you see this, please make sure you have started the game. If you have started the game, then move the game to the C:\ drive. To do so, see Moving a Steam Installation and Games

6. Once done, run the game!