Fisheye on nikon d5100?

Okay so I know that on a nikon d5100 the fisheye effect is built in an there's no need for a lens I was wondering how you put that effect on, I know how to put on the other effects but not that one any help? Please and thank you<3

A "fisheye effect" in software cuts some of the image from the middle of the sides and top and bottom. Then it squashes in the corners to create an extreme barrel distortion effect. This can never capture the actual wider view of a real fisheye lens on the camera.

The Nikon fisheye effect is a so-called retouch effect that you can apply AFTER you've taken the shot. The link below to Nikon's own site shows you how to do this.

Having said that, it's a digital effect filter, not a true fish-eye shot. True fish-eye lenses capture an angle of view of 180 degrees (some hugely expensive lenses can capture even more, and can look around the corner so to speak).
A typical kit lens for the D5100 such as the 18-55 mm lens, captures no more than 76 degrees at its widest (18 mm setting). The digital filter of your D5100 only applies the typical distortion that is associated with fish-eye lenses, not the actual angle-of-view of a true fish-eye lens.
In other words, it doesn't magically change the 76 degrees angle of an 18 mm lens setting into the 180 degrees of a fish-eye lens. For that, you need to invest in a true fish-eye lens.

I think it is in the playback menu, but I'm not 100% positive.

Fisheye Lens is not only "big nose" effect but also very wide angle of view. You can't see wider without real optical system.