This is more of an Excel question than a Powerpoint question, I think. So I give an Excel based answer, from the Excel help.

=SUM(C10:C25): current worksheet

=SUM(Year!C10:C25): other worksheet in current workbook

=SUM('C:\Reports\[Budget.xls]Year'!C10:C25): other worksheet in other workbook

But I'm afraid the changes in the one don't automatically show up in the other. You might have to use refresh (f9 or shift-f9).

Hope this helps.