Hmm, I might need to put in a readme then.
Anyway. Basically, yes, you do put the file you've chosen straight into Resources\Data. Use only one of the two files though, according to whether you want to see the items' stats described in detail or not. There won't be a problem if you use both, but in that case you'll have some items with the stats described, and some without. There is no "gameplay" difference between the two.
Also you might want to move "Items.itemsx" out of that directory and somewhere you'll remember where it is. It's the file that contains the default items, which my pack is designed to replace. Again, it's not a big deal if you don't, if you leave it there you'll just get both sets of items.
If you want to make sure it's installed correctly, go to the store and check the Armor section. The default items don't include armor at all, so if you see any you'll know you've got my pack.