What is the difference in these fisheyes?

0.42 and 0.18? I think.18 means you can see more then a.52, i'm not sure, please help! And if you have any fisheye links to buy one for the nikon d3100

You're right, 0.42 is closer than 0.18. But you're talking about fisheye adapters, not fisheye lenses.

Fisheye lenses are where the rounded effect of the photos (called distortion) is caused by the shape of the lens itself.
Fisheye adapters are like a second lens that screws on the front of a regular lens to turn it into a fisheye one. Generally these adapters aren't very good quality.

The numbers you've indicated are what the adapter does to the lens it's attach to. For example, say you have a regular lens with a focal length of 50mm. If you attach a 0.18 fisheye adapter to it (sometimes called 0.18X), the focal length will become 50mm multiplied by 0.18 (which is 9mm). This is super-wide. If you took a photo straight ahead, you'd be able to see your feet in the bottom of the picture. THAT'S how wide it is.
If you attach a 0.42 adapter to it instead, the focal length will become 50mm multiplied by 0.42 (which is 21mm). While 21mm isn't as impressive as 9mm, the shape of the adapter will still give you the distortion of a fisheye lens. You get the picture, if you'll pardon the pun.

The 0.18 adapter is more of a novelty. Something to make you go "Wow!" and have fun with.
The 0.42 is better suited for portraits, close-up subjects and general fisheye photography.

If you want to buy an actual fisheye lens instead of an adapter, Nikon make some really good ones (see links below). They're much better quality than the adapters… But bear in mind, they're a lot more expensive too. Unless you're pretty serious about it, I'd recommend the adapter instead.

Hope this has helped. Good luck and happy photographing!

PS. I don't think this question belongs in this category. Try Consumer Electronics->Cameras.