You have to choose Unrelenting Force in Shouts.
And you have to shout the second word in that shout I believe, so hold the key down for as long as possible.
I haven't figured out how to use the shouts individually. I guess you put them in favorites and then number them on hotkeys?
Put them in Favorites, press the Q button while playing to open the favorites menu.
Right click the shout or press E to equip it.
Alternatively, just select it in the magic menu.