You must be level 25+ in order to be offered this quest.
You, or someone in your group, must have completed or be willing to complete the quest, The Everling Lockets in Nektropos Castle. Killing the final boss, Lord Everling, is a necessary step for Hadden's Earring and to access him The Everling Lockets must have been completed by someone in the group instance.
A new page of Hrath's Journal will update with every step. Sometimes the text in the journal is explicit on where you need to go next and sometimes it's very cryptic. Below is the short and concise directions on where to go.
He is in the side room (behind the curtain) to the left as you walk into Workshop of the Sorcerer. When you go near the side room he will charge you and start attacking. Make sure to clear the main room first to prevent being overwhelmed by mobs.
Return to Kerath McMarrin in The Thundering Steppes.