Can you use calculated fields in Excel 2007 pivot tables when the data
source is an SSAS data cube?

I am connecting to a SQL Server 2005 data cube with Excel 2007 and viewing
the data though a pivot table. I want to create calculated fields in my pivot
table, but the "Formulas" button is grayed out.

How do I do create calculated fields (if it is possible when connecting to a
data cube) ?