First, go to View > Headers and Footers and be sure that the Slide Number item
is checked and click on Apply to All.
If that doesn't do the trick, go to View > Master > Slide Master > Format >
Master Layout. Is Slide number checked? If not, click on the check box to
select it and click OK. Now go to View > Normal. Do you see the slide number?
If not, post back here and tell us exactly what you did and what you see at each
step.