Opteka baby fisheye with nikon d5100?

I have recently bought myself a Nikon D5100, which came with an 18-55mm lens. I have also bought myself an Opteka baby death fisheye lens, the lens is a 37mm so I also bought a step down ring (55-37). Once i'd set up the fisheye onto the standard lens, I have tried and failed to get the camera to focus. Someone please help? Best answer will be awarded!

If you wanted to buy a cheap, pretend fisheye filter, you needed to buy one that had a 52 mm filter thread.

Mounting a 37 mm filter on a 52 mm step down adapter will only cause further vignetting

If you really want a fisheye lens (by definition on sees a full 180 degrees), you need to save your pennies and by one of these

http://www.amazon.com/...002KMAWO0/