HOW do you use the Atronach Forge in the Midden? I have all kinds of items needed, but what goes in that spot above the offering box? PLEASE HELP!!!!

You need to have a Conjuration Skill of 90. You need to complete the Conjuration Ritual for Phineas Gestor to recieve a Sigil Stone to place in the spot to make the Daedric Armors and the more powerful Ingredients.
You can make a few Scrolls, Staves, and a few Atronachs without the Sigil Stone.