Edit: I solved it on my own.
Solution:
Code below is correct, just forgot to name one thing.
Go to your key in the level editor and click the Entity tab.
At "CustomSubItemTypeName", enter your key name and it will work .
Well, I don't see where the problem is. My script looks like this:
<CATEGORY Name="Inventory">
<Entry Name="ItemName_celldoorKey">Cell Door Key</Entry>
<Entry Name="ItemDesc_celldoorKey">Tag reads "Cell Door".</Entry>
</CATEGORY>
Yet, no name or description shows up on the key in game.
Creator of The Dark Treasure.