Samuel said that the circus is the only way they make money and survive. So how do people spend money there if it's impossible to find?

I don't think it's impossible to find. But because they keep moving, people need the compass if they want to find it. It could be anywhere in the world. I think they make it visible for the people of the specific town they're in, once it's open for business. Though they do make it sound more complicated than that! ;-)

Most likely, the carnival is just impossible to find if they don't want you to find it. The compass just saves specials the trouble of researching town after town, circus after circus all around the world just to find it. It just points in the right direction and TADA! More like a GPS than anything.