I know it sounds obvious, but make sure that you are clicking the "Add Item" button after you select an item on the list. Make sure that all the files related to the items (mostly png/material files) are in your FeralHeart>>Media>>Items folder and that you replaced the Items.cfg (Located in just the FeralHeart folder) with the one provided in the legendary items pack.
Also check that you aren't scrolling over items by accident, as there are so many and the scroll bar goes quickly when dragged. A way to fix this would be by clicking just slightly underneath the slider bar so that you can move through the items on the list slowly.