In contrast to the review above, I recommend turning on your minikit extras as you'll be clearly shown where each minikit can be found. It also saves looking for a walkthrough. The minikit detector red brick can be found in the Lost in the Aether level and costs 250,000 studs to unlock.
In addition, you also have to complete this level with all minikits unlocked in order to unlock the achievement. I tried saving and exiting once I'd picked up the remaining minikit I needed and the achievement didn't unlock until I had replayed the level in full.