In the Magic>Power menu?

For example, I activate the Shadow Stone which means I can be invisible one time a day. How do I use it? When I equip it in Magic>Power, nothing seems to happen.

Thanks.

Equip it, and then press the 'shout' button.

Yep, Shout and Powers share the same button.

Tried that, nothing happened.

Have you remapped buttons?
Tried holding it?
You know the shout and power button are different from the normal attack ones?

Are you sure it's equipped?