Opteka fisheye lens and nikon camera flash?

I recently bought an opteka 6.5 mm fisheye lens and love it but…
in order to use that lens i must be on manual mode, which is fine, but that means that my camera (nikon D5000) doesn't detect the lens and won't let me use flash.
is there any way to override this setting on my camera( so it lets me use flash even if it doesn't detect the lens)
ps. I'm aware that because it is fisheye, the flash won't cover the whole photo
so are there any buttons i can press to make flash work?
Please Help! I'm going on vacation in a week and don't want to have to buy a flash, haha

You will need to go to Full Manual included the Flash. Study the Flash's user menu, it should show you how to turn off the Auto TTL function and should have a table and chat to show you the GN number as well as setting between f-stops.

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Opteka HD² 0.20X Professional Super AF Fisheye Lens for Nikon D40, D40x, D5000, D50, D60, D70, D70s, D80, D90, D100, D200, D300, & D700 Digital SLR

Included - .20x Fisheye Lens, Lens Adapter, 52mm Adapter, 55mm Adapter, 58mm Adapter, 67mm Lens Caps, Lens Pouch
An essential tool for extreme sports and dramatic action shooters
High-index / low-dispersion optical glass, Fully multi-coated, every element
Designed to maximize day and night shots by providing maximum light and image
Made and designed in Japan, Brand new, Lifetime warranty