As others explained already, I'm gonna help you too! ^.^
1. Open the downloaded items and go to Main Disk/C:/FeralHeart/media/items.
2. Copy everything containing MESH File, MATERIAL File and JPEG Image, PNG Image. If asks you to re-place press on 'yes'.
3. When your done go back to the main FeralHeart folder and copy from the downloaded items folder this: items.cfg
4. Place it in there. And if it says to you to re-place press on 'yes'.
5. Close all the folders and go to the game.
6. Press on 'Tools', 'Preset Maker'.
7. Go to IV and you should see all the listed items, press on any and see if you can see any of 'em.
8. Also, don't forget that when you press on a silected item, press 'Add'.
Good luck! ^.^