How do lens adapters work?

So, i'm new to the DSLR world, and i'm getting a nikon d3100 soon. It says it has an 18-55mm lens. If i wanted a fisheye adapter, such as this one:
http://photojojo.com/store/awesomeness/fisheye-lens/ which size or adapter do i get? DO i get the 55mm adapter?

Almost all camera lenses have a screw thread around the front of the lens for attaching filters and accessories like a fisheye lens. You need to find out the diameter of this thread.

It looks as if the 18-55mm lens has a 52mm thread, but do check. There are several versions of this lens available and they may vary.

By the way, those screw-on fisheye adapters are really terrible. They can be a bit of fun, but if you actually want good results you are better off buying a proper fisheye lens. Of course Nikon's own brand fisheye lens is pretty expensive, but there are third party options by companies such as Opteka. It's actually not a bad lens.