My fisheye lenses won't let me focus?

I try to focus and it won't let me… I have a nikon d3100.
or it wil focus but won't let me take the picture.

Added (1). They said that they ARE compatible with my camera.

Added (2). The company that made them said they are compatible. And it doesn't let me manually focus.

We can't help you without knowing what fisheye you have.

If it is NOT a Nikkor 10.5 mm, then it is probably a non-CPU lens and you will have to read in your user manual how to use them with your fine camera

Which lens are you using? Who said they were compatible? If it doesn't autofocus, it doesn't autofocus; big deal, just focus manually (which is easily done since almost everything is in focus all the time anyway).

edit: Which company would that be? Sounds like you bought one of those cheapie fisheye attachments that connect to the front of a "real" lens. In that case, next time you'll know better and save up for a proper fisheye lens.