attempt to index upvalue 'Torso' (a nil value) what is this?

You obtain the mouse using Player:GetMouse(). Then there is a property within the mouse called Hit that will get the 3D position the mouse is pointing at in CFrame form. With that information you can reset the position of the brick to the Player's mouse when the player clicks using the Button1Down() event.

http://wiki.roblox.com/index.php?title=API:Class/Mouse <- wiki

--In a local script

player = game.Players.LocalPlayer

mouse = player:GetMouse() --Get the mouse from the player

local part = PathToPartHere --part you want to move with the mouse

mouse.Button1Down:connect(function() --Mouse button is clicked
    part.CFrame = mouse.Hit --Set the new brick position to the mouse hit property      
end)

--This should work. Use Remote Functions/Events if FE is enabled to change the position of the brick