Putting Nikon D5100 on Auto ISO on Aperture Priority mode?

How can I set the ISO setting to "Auto" when I'm in Aperture Priority mode?
It keeps telling me "This option is not available at current settings"

If you have selected a small aperture, the camera might not let you select Auto-ISO if the shutter speed would be slower than the one you have specified as your minimum shutter speed. This is sort of a guess on my part, because I don't know what other parameters you may have selected that would exclude the selection of Auto-ISO.

Try reading the sections in your Owner's Manual about using Aperture Priority and Auto ISO. This may also help: http://www.nikondigitutor.com/eng/d5100/index.html