I'm in the same position. I thought I might have accidentally sold or dropped it in a rush, but after paying almost every NPC a visit and after retracing my steps across the map, I simply cannot find the scroll. I checked all over her house and through all of my groups inventory, but I've had no luck.
Is their any other way to access her secret lair? I have absolutely loved the game so far, and after putting in 15 hours I really would loathe having to start all over again.