The path ahead is difficult, <name>. We must devote the entirety of our mental power to the task at hand.
Yet sometimes, that is not enough. Sometimes the situation calls for raw firepower.
I know just the spell for such a situation and I'll be more than glad to share it with you. Once you've learned it, practice using it against the Bloodfang Worgen to our west. You'll only get the chance to use your new spell after casting a different spell.
This is the introduction to the class trainer system and how Arcane Missiles work. Talk to Myriam again and purchase the spell.
As the quest text hints at, Arcane Missiles is a reactive ability and can only be used when it is available after casting other spells. At this point, the only other spell available is [Fireball]. Open up with a Fireball and keep casting it on worgen until Arcane Missiles procs. Then use Arcane Missiles to complete the quest.