I have a Nikon D3000 and a Rokinon 8mm fisheye. Dark photos?

I have to put the camera in manual mode, I open up the aperture ring all the way to 3.5, my pictures are dark unless I set shutter speed to slower than 1/2 sec. Makes them blurry at this point.

Those fisheye adapters are "you-get-what-you-pay for".

By all means, why don't you increase the ISO to whatever your camera can deal with and reshoot the scene.

You probably have your D3000 set for a low ISO. Change your ISO to something higher, such as 800.

First, to address the previous answer, the Rokinon 8mm fish-eye is a lens, not an adapter

This lens is a fully manual lens for Nikon mount so you will need to put your camera into whatever mode you need to so you can use the aperture ring on the lens. You will have to adjust your shutter speed manually to get a proper exposure. If you need to adjust the shutter speed to something that is too slow for whatever you are photographing, you will need to increase your ISO setting. Manual lenses can be fun but they are definitely more of a challenge to use correctly than auto exposure/auto focus lenses.

That's normal. This is because the Nikon D3000 doesn't support metering with the Rokinon 8mm f/3.5 fisheye.

The only mode you can use the lens in is in M mode, using a guess-and-check method to get to proper exposure. If your shutter speed is too slow, either move to somewhere that has more light, or raise your ISO setting.