You find the Stud Magnet red brick in the Carnivore section of the Jurassic Park HUD world. To unlock the red brick you need to help a park ranger find 3 lost eggs.
You'll need a custom dinosaur (I used a velociraptor with a t-rex head) to smash a piece of amber with your roar, another egg is hidden behind some reeds, and the last one requires you to break a machine on the side fence with a raptor, then use a female character to do some gymnastics and swing over to the egg.
Once you have the red brick, you go back to the Visitor Centre and unlock it for 100,000 studs. In the pause menu, activate the Stud Magnet, and the achievement will pop.