Operating system? Do you mean the dedicated numberpad area,or the numbers above the alphabet keys? Do you have the numlock on?
What are you trying to do, and what happens when you do it?
MacOS 10.6.8 dedicated numberpad.
Trying to enter numbers in a text box in InDesign CS5.
Numberpad works in other programs.
Is this numberpad built intothe keyboard, or an add-on? You didn't say what happens when you try to use it.