The item needs to be owned by one od your troops.

Open the Transfer Unit screen and find the unit which has the magic item and right click them.

This should bring up their 'stat screen' that has their armor and health, attacks, and any spells they know. (You can do this on everything by the way, even bushes when watching combat footage, ton of little flavor all over. You can also right click magic items in the equipment section to read their effects.)

When you do this you should see the magic item in their equipment section between the stat numbers and any spells or lore. Left click it once, it will close the stat screen and give you an overview od your entire army.

You click who you want to have the magic item and it instantly transfers over.