After installing, make your way to the new gold location on your world map, Bann Loren's lands. There, you'll meet another survivor of Ostagar and he'll tell you about the key he left to King Cailan's chest. Head back to Ostagar and fight through the Darkspawn to gather the pieces of Cailan's armor, the contents of his chest, and Duncan's weapons. You'll have to fight through the lower level of the Tower Ishal again, but after going through a tunnel, you'll emerge onto the battlefield. There the Ogre awaits. He's no tougher than any other ogre you'll face, so treat it no differently. After defeating what Darkspawn are there and the Ogre, this will unlock.
As for Cailan's chest, I'll describe the location for you until I can find a suitable map. On the western side of the Ostagar bridge (where you start this DLC set) is where the chest and the key are. The key is just to the north of the bridge entrance in a pile of rubble. The chest is to the south of the bridge entrance in between two spiked, wooden walls.