This quest is given by Deputy Kegie in The Enchanted Lands at -207.41, 0.40, -353.70 Copy after you complete the Sanity Check quest.
You must kill 15 Mistcaller Mischiefmakers in The Enchanted Lands. They are found in the water near the fairies.
After doing this return to Deputy Kegie to open up the "Stolen Jum Jum" quest.