How to see page breaks in Numbers version 3.x
Unfortunately, Apple's Numbers has never had page breaks.  Here is a solution created from Yellowbox on the Apple's Community forum: https://discussions.apple.com/docs/DOC-7101.  

Here is also a answer from Ian, also on Apple's forums--

"Numbers has never had Page Breaks.

If you are using Numbers '09 it has Show Print View which is useful for arranging (and resizing) objects on pages before you print to paper.

If you are using Numbers 3, there is only Menu > Print > Print Preview (Print Setup) which shows the pages, but is not editable."