Some low-end or older cameras don't show the ISO in the viewfinder. This bothers some photographers because it forces them to take their eye off the viewfinder to change their ISO, especially in manual mode.
Such is the case with my D70 but it doesn't bother me. That's because I don't pay attention to the number of the ISO so much. Instead, with my eye in the viewfinder, I just adjust the ISO and watch the light meter and/or count clicks of the command dial. As fellow blog author mshafik paraphrased Neil van Niekerk, "the exact shutter speed & aperture settings are not the important part, it’s the thought process that results in a certain exposure."