Do not install the DLC onto the full game purchased via Denpasoft - it already has it installed.

1. Make sure you have installed the game via Steam. 

2. Download the DLC via Denpasoft (in your account's downloads). The file will look like this: 

3. Open the 'nekonin3_ec_fullpatch' folder to see: 

4.  Go to Steam, and right click on Neko-Nin exHEART 3. Then go to Manage and Browse Local Files. You should see the following:

5. Clicking on 'Browse Local Files' should show this:

6. Copy the files from the downloaded patch (step 1) into this folder. Confirm the replacement of files. It should look like this:

7. You're done!