Opteka.35 fisheye very blurry?

I'm using it on a Nikon D4 (Full frame), to which it should be compatible. I attached it to my 50mm 1.4 lens. (Which in FF should be like a 35mm on a crop camera). I have tried switching to manual focus and the view is COMPLETELY blurry. (Not just bad quality blurry, but 100% blur). What am I doing wrong? The macro attachment works fine.

Added (1). Thanks for all your responses. Yes, I was also thinking that maybe my 50mm wasn't wide enough. I'm planning on getting a 35mm in the next couple months, so will try again on that. In the meantime, the macro works great. And yes, I know this isn't a lens, it's an attachment with shoddy glass on a gorgeous 50mm 1.4G lens coupled with a $6, 000 camera. I wanted it for fun to take some interesting pics of my kids. I wouldn't be using the fisheye for professional use unless I had an actual fisheye lens (a little outside my budget right now, since I'm eyeing that new Sigma 35mm 1.4). But it's on my "win the lotto" list.
I trying closing up my ap, but that didn't help. At least the macro works, so I may not return it.
I just jumped the Canon ship for Nikon, so trying to build up my lens arsenal again, just can't afford a "real" fisheye yet.

Added (2). Wow, didn't think I would get such a hostile answer to a simple question.
As mentioned previously, this was just a fun accessory to get some interesting pictures of my kids during off hours. In fact, I know several professional photog friends who own D4s and D3's and use macro tubes and adaptors and fisheye adaptors for fun. They were the ones who recommended this item to me. I don't think there's ANYTHING wrong with putting a cheap fisheye adaptor on a $6, 000 camera as long as one understands it will not be of the same quality as the lens itself (or an actual fisheye lens). And secondly, that one does not use it professionally, charging clients for images that are of much less quality that what they expect.

1) you need to assure that you have the diopter in the viewfinder adjusted for your eye
2) use the electronic rangefinder in the cameras viewfinder to assure sharp focus
3) When using the Opteka fisheye, unless it is an auto-focus lens, you will need to use the electronic viewfinder.
4) Opteka makes a 6.5 mm and 8 mm fisheye lens, but not a 0.35 mm fisheye lens.

NOTE: I hope you are not trying to use a $70 0.36x add-on filter to convert your 50 mm f/1.4 lens into a fisheye. Usually by the time a person buys a $6, 000 camera, the know the difference between an add-on filter and a true fisheye. A fisheye lens by definition provides a 180 degree view. Add-on filters usually only distort the image but can't provide a full 180 degree view.

Yes that's right. They are! They are designed to be fitted to wide angle lenses not to standard or telephoto.
I have one (bought for 50p from a junk sale!) and the only lens it produces half decent results on is my 18-55 kit lens at the wide end.

What aperture are you using? Does it sharpen up with a smaller aperture?