First of all, don't replace the items.cfg. Simply open up the .cfg file (the item pack you just downloaded) and copy the codes.
They should look similar to this:
[Christmas Hat]
mesh=hat1.mesh
bone=forehead
offset_f=0.1 1.2 0.5
rotation_f=10 90 -10
offset_c=0.1 1.4 0.1
rotation_c=10 90 15
And head back to (C:) Local Disk > Feral heart and open up the item.cfg file. Paste the codes at the bottom of the file, hit save, and boom. The items will appear in-game. However, remember to place the other files (ex. .png and .mesh files from the download) in the correct spots, or else only the name of the items will appear in-game, but not the actual item itself.
Hope you understood this.