You obtained the mithril nugget and the ruined key as Dwalin asked, but his smith, Litli, informed you that there was still one more thing needed to work the mithril.
'To work with mithril, you need a special hammer. So special, in fact, that we only ever had one here in Thorin's Hall: Thrór's Hammer, the hammer of Thorin's grandfather, which Thráin brought here! Of course, since we never had any mithril to work, we never used it.
'When most of the Longbeards moved back to the Lonely Mountain, the hammer got left behind, and -- as an insult to us, I'm sure -- the Dourhands took it and placed it in Skorgrím's tomb as tribute to him.
'At least, that is what one of them told us. We tried reclaiming that tomb, but it seems cursed. Vile things started lurking in the shadows there. I would say we should just let it lie there, but this task of yours is important enough to warrant opening the tomb again. You'll not find any dwarves willing to go in there, so you will need to retrieve the hammer yourself. The tomb lies south of here, in the Berghold.'
Thrór's Hammer lies within the tomb of Skorgrím Dourhand. The tomb lies south-east of Thorin's Hall.
Dwalin told you he needed a hammer that Thrór, Thorin's grandfather and last King Under the Mountain before the coming of Smaug, took with him into exile and which Thráin his son brought to the Blue Mountains.
Litli can be found by the forges near the throne of Thorin's Hall in Ered Luin.
You should take the hammer you found to Litli so that he can forge the key you need.