My 8mm fisheye lens won't work on my camera?

I have a Nikon D3100 and my dad bought me an 8mm fisheye lens. When I put it on the camera wouldnt let me shoot with it because it couldnt detect the lense. But when I put it on manual it worked but it looked terrible. I would like to take pictures with other settings other than manual.

If its the Samyang which is marketed under many names then it has no electrics and is a manual focus lens. Don't let that deter you. Is this it? Http://www.photozone.de/canon-eos/526-samyang8f35eos

It is a low cost lens. There's nothing wrong with your camera. Your camera (as well as the $6, 000 Nikon D3s) will only recognize the lens when your camera is in manual, as it is a manual lens.

You have to learn how to use it.

If you want to use the lens without having to use manual exposure controls, you will need to spend $800 on a good fisheye lens.

But since you have a DSLR, one of the advantages of them are the manual exposure capability, as sooner or later, you will not be able to get the photo you want without knowing how to do this. So use this as an opportunity to learn how to use the advanced features of your camera.

If it works in manual mode then that means it works. You just don't know how to use it. Learn photography. The easiest way to start is to learn the Sunny f/16 Rule. Research on it.

Sounds like you are going to have to learn to use manual setting. But hey, that's how you're supposed to use a DSLR camera anyway. Only posers don't know how to use manual.