You can follow this procedure :::

Lets assume revenue is the field in which we have to display null values as 0.

First create a calculated item and

Then write the expression of the cal item using If Then Else construct as

If (Revenue is null) Then ('0') Else(Revenue)

I think u can easily understand this.

Now u can see that the null values r displayed as '0' in the calculated item.

Select the data item in which you want to replace null with 0 .Go to Properties pane, Click Data Format, in this put 0 in Missing value Characters.