As soon as you get to Cape Solace, speak with the gnome to begin this quest line. You will want to watch for any item labeled "Bundle of" while looting, as these are the materials he will ask for to upgrade. Each section of the keep will require a 15-20 minute wait in real time (or just sleep for 24 hours game time) before the upgrade is complete and you can turn in the next set of materials, so start this early and keep checking back while you do the main quest line. The final quest will not be available until you've stolen The Requiem during the main quest.
If you don't have the materials he needs right away, be sure to make this quest your active one and your map will be marked with loot locations that have what you're missing.