One way of doing this is using a combination of GotoSlide and SlideIndex
for page down
ActiveWindow.View.GotoSlide ActiveWindow.Selection.SlideRange.SlideIndex + 1
for page up
ActiveWindow.View.GotoSlide ActiveWindow.Selection.SlideRange.SlideIndex - 1
you have to add some code to make sure slide index remains in its proper range
something like
if ActiveWindow.Selection.SlideRange.SlideIndex <
Activepresentation.slides.count then
ActiveWindow.View.GotoSlide ActiveWindow.Selection.SlideRange.SlideIndex - 1
.....
etc ..