Note: Dire Wolf and Dire Ravager do not work for this quest.
Note: In spite of the quest text, you do not have to actually bring the Dire Warg back to Wolfmaster Heinrich. Simply getting the muzzle on the animal seems to satisfy the quest. However, you do have to make it back to him without dying, because that will break the 'charm' that the muzzle casts on the wolf. If that happens, Heinrich will say something about you having some trouble, give you another muzzle and ask you to try again.
Does this actually work? I have tried to tame 4 dire wargs (3 epic and 1 normal) and I get no option to use the item on the dire wargs.
(answer) Yes, I found that I had to equip the belt in a charm slot. Then open my character sheet (C) and select the 'equipment' tab. Right click the belt (charm slot) and select the 'use' option. It's a hassle but it works. If the wargs are white or yellow to you, or if you're a mage, you need to be fast because you need to be in agro range for the charm to work. Also, it takes a couple sconds to cast. I recomend opening your character sheet before aproaching the warg. The muzzle will not work from a hotbar.
5/13/16 you now have a button in your quest helper. click it once to equip and again when in aggro range to muzzle.