EDIT: This is part of the Dawnguard quest line, the actual mission where you can start learning Soul Tear is "Beyond Death"
Each time you summon Durneviir you will get give one word to the "Soul Tear" shout. So you will need to summon him 3 times. The shout takes 300 seconds to recharge, so 5 minutes; just wait an hour and summon him again ( Due to a message I need to specify waiting using the in-game "Wait" feature) . Try summon him in a wide open area so it doesn't glitch, sometimes I found him to spawn and insta-die.
You do need the third level of "Summon Durneviir" so you will need to spend at least 3 souls on unlocking all 3 levels. NOTE: You don't need to unlock any of the shouts for Soul Tear using Dragon souls. Simply having learned them is enough.
EDIT 2: Make sure you aim the shout at the ground to summon him. You will need some clear space within which to summon Durneviir. The planes outside Whiterun are ideal, although I did it out the front of Castle Volikhar