Samyang lenses on D3100?

Is http://shashinki.com/shop/samyang-500mm-telephoto-mirror-lens-samyang-mount-nikon-mount-included-p-5966.html and http://shashinki.com/shop/samyang-500mm-telephoto-preset-lens-samyang-mount-nikon-mount-included-p-5964.html#!tab3 compatible with the Nikon D3100?

Yes it mount mount onto the Nikon, but it will be manual focus only. Also be aware that it is a fixed aperture mirror lens (no adjustment) - mirror lenses also tend to produce 'donut' shaped bokeh, which some love & some hate.

I have never even heard of Samyang but yes, they provide an F mount for Nikon and your D3100 is the F mount (as all Nikon's but the underwater models, are).
.
.