Where can I find the minikit detector in the Middle Earth?

The Red Brick "Minikit Chest Finder" is in the area north of Weathertop that looks like a crypt (might be their interpretation of the Barrow Downs). You'll need a character with strength to pull open the orange handle leading to it across the river. Once inside, you'll find a hobbit who is asking for a "Mithril Stud Mallet", the design for which can be found in the Dead Marshes.

To find the design, go through the Dead Marshes until you're at the last area where the Nazgul chases you, then find a small LEGO dock to fish off of. You'll pull up the design.