linked files
 

I had a DSUM formula in Excel 2003 that summed data from one file based on
criteria within another file; DSUM([Sales.xls]Billings!$A$:$Z$,15,$A88:$A89).
This worked just fine in Excel 2003! Now that I am using Excel 2007 it is
not working...what am I doing wrong? I tried updating the links (even though
the source file is open at the same time), and updating the calculations,
however, the cell remains blank.....HELP