Photoshop 5 question

Greetings !

I have this peculiar problem :

To adjust the contrast or tonal range of the image, We first go to
Image >Adjust > levels and accordingly move the sliders, after making the changes recheck 'Image> histogram and verify the graph .
Now my problem is that the dialog box under levels is black and does not show a graph due to which everytime I adjust the contrast of the image I have to look at the graph in the histogram and then getting the values to make the alterations in the levels command.

I even verified this in the tutorial and the levels dialog box is supposed to show a graph. (there is a box ???)
Could someone please guide me .