How do I use Extracted Legendary powers?

You select one of the three slots you want to add the power to. Then you select the item whose power you want to use. The item is then visible in the selected slot. Once its visible, the power is active.

This means you can have a total of 3 powers active. 1 weapon, 1 armor and 1 jewelry.

The easiest way to test the cube is following:

1. Open the cube
2. Select jewelry slot (ring)
3. Select "Convention of Elements"
4. Close the cube
5. Look at your buffs and see "Convention of Elements" active

Also, the selected items in the cube are tied to each armory set slot. So if you set up 2 armory sets with different cube items, there is no need to change the cube items manually if you change the equipment set via armory.