I've been building a few Project Server 2010 reports with Report Builder 3.0 lately and have had to deal with missing values in iif statements. Unfortunately you can't test for the missing data in the iif statement as it evaluates all the expressions first so you need to test for missing values earlier.
The easiest way to do this I've found is to use code in the Report definition. Right-clicking on the body of the report (not the table or chart) brings up the report properties. Clicking on the Code tab brings up a box where you can enter your custom code.
In this case I want to make sure that at least a 0 is returned so I wrote this function
public Function NullToZero(Byval x as long)
If not x > 0 then
else : Return x
Then simply use the function in your expression like this: