To set aperture on a D5100 switch to A and use the command dial.
(Shooting in Manual mode is simply torturing yourself unnecessarily if you don't know exactly what you're doing. In Aperture priority — or Program shift — setting aperture is simple and intuitive.)
In manual mode it's not that simple from what I could glean (haven't touched a D5100 myself). You have to half depress the shutter release, simultaneously press the exposure compensation button and only then you can adjust the aperture with the command dial (otherwise you'll just change shutter speed). That's pretty crummy IMO and rather elaborate for a beginner.
My D7000 is more straightforward since it has a separate dial for this and no need to half depress the shutter release. (I assume you can also change the settings using the LCD, but that's of no use if you're looking through the viewfinder and trying to set manual exposure.)
Looking at the manual you apparently don't have to half depress the shutter, just the exposure compensation button (which has an aperture symbol next to it), but other websites suggest you do have to. I'll look into it when I can see the camera in person in a few weeks.
Why not? Every pre-digital photographer did manual exposures. It teaches you to pay attention to your environment, and ultimately, I believe, makes you a better photographer because it develops your intuition about a given scene and what does and doesn't work.
I started off with P&S cameras and before I knew the mechanics of cameras I was frustrated with the automatic exposures; once I discovered I was missing "manual mode" I never turned back.
Sure, from time to time I'll use the priority or program modes when the shot is more important than the learning experience, but after 4 years of hobbyist photography I still find myself with much to learn and I enjoy the challenge of manual exposures.
I started out with manual everything cameras processing my own film, and I got pretty darn good a judging lighting by eye. Unless what you're trying to learn to do is become a human light meter, the "education" you're describing is pretty silly. You can accomplish the same results more quickly using the correct automatic setting and exposure compensation.
To set aperture on a D5100 switch to A and use the command dial.
(Shooting in Manual mode is simply torturing yourself unnecessarily if you don't know exactly what you're doing. In Aperture priority — or Program shift — setting aperture is simple and intuitive.)