Nikon D80 Help. How to get the flash to work with manual lens

I just recently bought a Rokinon 8mm Fisheye lens for my Nikon D80.
I love the lens, it works perfectly during the day time, when I require no strobes, but when I attach my External flash, the shutter won't release, and the flash won't fire. On the top LCD screen i get the flash symbol blinking.

I have tried everything, changing my settings on my camera to allow manual flash, commander flash, and then standard flash, yet nothing works.
Any ideas, or help would be greatly appreciated! Thanks

Added (1). @fhotoace

Firstly, you are very rude. Secondly, I have you know that I have been taking photos with several DSLR's and SLR's for over 5 years now, covering weddings, events and modeling shoots. And finally, the reason I need an external flash to fire is because it is shooting into a diffuser, which will evenly spread the light.

So, get off your high horse, stop thinking you are the 'bees knees' in photography (cause quite frankly, you fail the first thing a photographer learns, and that is to be tolerant, patient and nice) and get a life.

Nikon SB flash? Set external flash to Auto mode, lowest focal length and ISO. Select distance range. Take note of aperture size. Set camera to manual mode, maximum flash sync shutter speed, same ISO as flash, and aperture size recommended by flash. The flash may not be able to fill the whole angle of view of the 8mm lens. It's just is too wide for one single flash.

It is clear that you have never taken a class in photography,

There are NO flash units that can cover the 180 degree view of a fisheye, NONE

Your camera is telling you something. A flash can't cover the whole field of a fisheye lens. This would have been clear to you had you taken at least one class in photography

Have you changed the setting on the flash unit from TTL to Manual?

If you'd specified the flash you're using this would have been easier.

I'm 100% sure you manual lists the available flash modes in Manual mode. Look at that section