To face Frostmaw, you must entice him with the meat of his favorite prey.

Go to the Alterac Mountains in Azeroth and hunt a hulking mountain lion. Kill it, and then take its carcass to the Growless Cave, a place held sacred by the bestial wendigo.

Place the carcass on the Flame of Uzel and the scent from burning meat will drift from the cave.

And then, in time, Frostmaw will come.