I don't have the plans in front of me and have to do a little guessing based on memory and what photos of my build that I have available. The L2 parts form part of the rubber band motor system by providing a secure mounting for the rear motor peg. If you're not going to fly on rubber power these can be safely discarded. If you want to use them they go just above the side keels on both sides just in front of fuselage former B8. You should be able to see this on the side view, just below the antenna and partly covered by the star and bar.
you can see it here. note i moved mine up one former, but in theory its the same. hope the picture is clear. a lot of times the L2 parts are a poor fit, so some sanding may be necessary. also, the one in my build here are actually backwards. the hole for the dowl should actually be facing the rear of the plane.