She can be found North Western part of Kavatch by the lake
http://i.imgur.com/3jf47w4.png
Below is what I think you need to do to meet the requirements to unlock access to this quest (sweetroll killer)
Complete the dark brotherhood main questline
Complete every side quest (Non-daily) except for this one
Reach Rank 9 in the Dark Brotherhood Skill tree (Less sure about this one, took me a while to find the location of the side quest before this, so it might be unlocked earlier)