Pressing "4" will activate it to be used. You have to push whatever button/key is set to use your shouts.
Edit: From: Game-Xpert | #002
It should be Z.
Thanks. I use a controller and immediately changed the controls around to suit my tastes, so I wouldn't know the default for either the keyboard or the controller. :/ --- Mr. Spock, the women on your planet are logical. That's the only planet in this galaxy that can make that claim.
This is just a wild guess... Even if you did set the keystroke to your taste... did you equip the shouts themselves? --- I just changed my personal arsenal to fifteen Metal Gear Ray...... And I'm looking for a target. Who wants to be one? You?