I believe the page up and down buttons used to work in ComputerCraft. They currently do not seem to be working. (CCv1.6.4) Tested with: while true do print(os.pullEvent()) end

@DeGariless Close the NEI interface and try again. Its most common reason for those key presses not working. By default to close you press o key with your inventory open.

Page up and page down work just fine. it's possible another mod is interfering