Once you reach 100% (collected all the 250 gold bricks, 16 red bricks, 160 minikits, 16 true heroes, saved all the Adam in Perils, unlocked and purchased all 147 characters and vehicles) go to the Watchtower Control Room and head to the right doorway by the globe (where the Green Room cheevo pops). In the center is a new switch stand, pull it and the studs start raining down.