After tiresome search for ninja's appereance in many community lots, seeing him dissapear before my sim could get to him, and then (of course) answer the wrong question - I got pissed off.
So, go to the Far East Vacation (if you can buy a vacation house - good, but not necessary).
Choose one of the community lots, pass to Build Mode, and put Inge's Teleporter Shrub (it's the only one you can put in a community lot) in some spacy place.
Take your sims there, and click on the shrub; choose: "summon", then "more", than "ninja". Click on him, and "learn to travel like ninja". If you answer the wrong answer - summon a ninja again, until you answer the right answer. He will wisper something in your ear - and voila - you have the Memento...
.
It's much easier, if you have a vacation house. Put the shrub, and do it comfortably, without any tense of losing motives.
Get the shrub from here:
http://www.simlogical.com/sl/Sims2Pages/Sims2_Teleporters.htmStupid EAxis!
Edit: Download the Teleporter Plus Shrub.