At first I just left the Nori in the tank, but since the urchins are blind, by the time they would find it (if they did at all), the Nori had already rotted (which of course polluted my tank). So what I started doing was sticking a piece of Nori between the Urchin and the surface it was on. Eventually the Urchin "felt" or smelled the Nori and would grab it when I put it next to him.
My pencil urchins also eat leftover frozen food (shrimp, mysis, etc.). I thought all urchins were herbivores, but apparently the pencil urchins are omnivores - which might explain why they ate the featherduster (I had the Sabellastarte sp. - see Saltwater Aquarium Invertebrate for Marine Reef Aquariums: Feather Duster Fan Worm
). The longspine urchin is a herbivore I think, so you might not have a problem with featherdusters.