crosstab nulls

[Dugantrain]

I have been searching through this forum and understand that the Nz function should be used to handle nulls in a crosstab, which is exactly what I want. However, where in the query should this function be placed? Can I do it in query design or should it be expressed in the SQL statement? For the record, I have a crosstab which shows kits as row headings, items in these kits as column headings, and the sum of the Need To Order where they intersect. In the query that this crosstab is based upon, I have set the Need To Order criteria as >=0 so that it will not capture surpluses in the kit (which would be a negative number).