I remember ASA from when Kodachrome had an ASA of 10. ISO means something else in the computer world.
I spent last summer shooting my D5100 in some incredible places. I had to learn the camera quickly before the trip. I shot both video and stills at the same venues. I found that shooting in Aperture mode was the most productive. If you shoot video, I think you have to shoot in Aperture mode.
I remember adjusting the Aperture with the command dial, and the ISO by pressing the Fn button on the left side (After setting the Fn button to control ISO) and then adjusting ISO with the command dial while making sure that the shutter speed was in the proper range. It is a little cumbersome, but it was the best I could do. The controls on the D5100 are a little different than what I was used to on other Nikon DSLRs. I found that neither eagles nor bears liked the sound of the D5100 shutter.