Hmm... Are you absolutely certain that you put the items.cfg file into your FeralHeart folder (not Media, not Items, but just the FH folder)? And, everything else in the download should go into FeralHeart>>Media>>Items.
And, of course, when you go to place all of these items in their folders, be sure to click 'Replace' for all of them. If they appear invisible, it means that the .cfg file was alright, but something's up with the images/meshes for the items themselves. If this persists after you've double-checked, I would suggest re-downloading the item pack altogether, and then trying this again.