Head back to Nivorajean who will ask you where you found the floorplans. He will first show you the map on which the treasure chests are found, and then give you a listing of locations from which you must choose the <pos> where your treasure chest was. You may choose to take another look at the map, by selecting "Think a little longer." but you cannot cancel the NPC dialog unless you forcibly disconnect (and you will presumably keep the Piece of Ripped Floorplanskey item).
WARNING: This quest (as of Oct 9, 2006) still has at least one bug in the offered chest locations:
Valid chest-location (L-8) is not offered as an option, but (K-9) is the "correct" response for that chest.
After one game-day, if you chose the correct <pos>, he will reward you with the Map of the Sacrarium.