What do the instructions say?

What if you open Excel before attempting to edit data?

Katrina:

I think you and I have discovered a bug in PPT 2007.  Editing data in a
chart does behave quite strangely and differs if you use the Edit Data icon
under the Design Tab when the chart is selected as opposed to selecting the
chart, right clicking and selecting edit data.  When doing the latter, at
least 80% of the time Excel opens up a totally blak spreadhsheet as in no
lines, no nothing, even though the chart being edited is populated with data.
The only way out of this is to click within the slide itself to close the
Excel window, then right-click Excel in the taskbar and select close.  Then,
select the chart again, go into the design group tab and select the edit data
icon, which works 99% of the time.  When it doesn't, I simply change to
another slide, come back to the one I'm trying to edit and use the edit data
option in the design group.  It's a huge PITA but it's working for me.