Sigma lens for Nikon D50

I have just purchased a Nikon D50 body only and a Sigma 28-300mm
F3.5-6.3 DG MACRO lens for it.
When I am in Manual mode (S, A, M) the selected focus rectangle is on
the right of the screen. Is this the focus area? I cannot change this
focus area rectangle to one of the other rectangles as the options for
changing focus area are only available in AF-Mode and then only Single
area Dynamic area or Closest subject.
Grateful for any help, Nuala

Thanks - The four arrow button worked but only when I had just switched
the camera on. It seemed to stop functioning as soon as the speed and f
stop appeared in the LCD screen. It is all so new that I am sure I am
doing something wrong. I feel like I need four hands at the moment to
turn on and off all the features.
I bought it in France and they sent a German manual and my mother
tongue is English so that did not help. I now have a manual in
English...

Thanks - The four arrow button worked but only when I had just switched
the camera on. It seemed to stop functioning as soon as the speed and f
stop appeared in the LCD screen. It is all so new that I am sure I am
doing something wrong. I feel like I need four hands at the moment to
turn on and off all the features.
I bought it in France and they sent a German manual and my mother
tongue is English so that did not help. I now have a manual in
English...