This quest/content is included in the Signature Edition of Dragon Age 2. It will be automatically unlocked at any point of the game or at the beginning of The Destruction of Lothering quest although its only possible to complete this in Act 1 onward.

To complete this quest during Act 1, you have to go to your Uncle Gamlen's Home in Lowtown  and enter the southern room. During Act 2 and 3 you can find it in the northern room in Hawkes Estate in Hightown. From there you can find a staff named Staff of Parthalan inside the chest marked as Special Deliveries. Upon taking the item, you'll complete this quest and receive a codex entry Staff of Parthalan.