Excel 2010 PowerPivot: How do I show items with no data?

Actually this is a better solution. Thanks to Alberto Ferrari for this one.

You need to build a Months table (ie a list of month names, Jan/Feb/Mar etc - a linked table will work just fine), and create a relationship between your new Months table and your fact table.

Then write a measure like this one:

NeverBlankUnits:=IF( ISBLANK( SUM(FruitSales[Units]) )
                      , 0
                      , SUM(FruitSales[Units]) 
                   )

EDIT: When you add your new months column into your pivot table, you may find the default sort is frustratingly alphabetical; Apr, Aug, Dec, Feb... Here's a great tutorial showing you how to get round this.

A workaround might be possible to use a Data Analysis Expression (DAX) to replace blanks with zeroes.

DAX can force a zero instead of a blank whenever no data exists in the fact table but you want the row to appear in the PivotTable.

For a simple worksheet with Depts, Divisions and a fact table called Numbers, and with a single column "D" containing a number to sum, write the DAX code in this way:

=IF (
    COUNTROWS (Divisions) > 0; 
    IF (ISBLANK (SUM (Numbers[D])), 0, SUM (Numbers[D]))
)

For more information see :
How to Use DAX in Excel's PowerPivot Add-In
Data Analysis Expressions (DAX) in PowerPivot

This approach was originally suggested in the thread Show Items With No Data on Rows.

A brute force method would be to create a measure for each month. As long as one of the month columns has some data, all month columns will appear. Creating all these individual measures is very tedious though - not ideal.

=CALCULATE(
   SUM([Units])
   ,Filter('FruitSales',[Month Name]="August")
   )

this works for me:

=if(sum(calender[Date])>0;if(isblank(sum(Sales_SalesOrderDetail[LineTotal]));0;sum(Sales_SalesOrderDetail[LineTotal])))