Movies

I use the movie setting often and sometimes I use it at night or in a room that is dark. Is there a setting on the 5100 that will allow the movie to be brighter or is there a light of some kind I can use to make it brighter?

you can get LED lamps that mount on the hot shoe

something a bit like this

CowboyStudio VL-76 LED Ultra High Power Light 76 LEDs: Amazon.co.uk: Electronics

I still don't understand weather ISO affects the movie mode. I don't use it that often so I guess that's why I don't know

one way to find out. Go in a dark room and shoot a movie at ISO 200 and then shoot the same think at ISO 800. Play it back and see what happens.

Obviously, lol. I just don't use the movie function much. I'll upload youtube vids very occasionally, but that's every other blue-moon or so.
OP, try a higher ISO? Faster glass? Lower your F-stop?

Always remember, you didn't buy a video camcorder. You bought a camera with the ability to shoot movies in a very limited way.