I have a sheet where I load values based on selections from dropdown
boxes. I also have the option of Drilling down on cells which will
display children below the parent cells. When I do a drill down, the
cells that do not have a numeric value change to zero and immediately
update to be blank cells. I want them to be displayed as zeros. How
can i do that? The format of these cells is 'custom' format!!!

It sounds like you may have viewing zeroes turned off. Click on
Tools/Options on Excel's menu bar and click the View tab on the dialog box
that appears... do you have the "Zero values" check box in the "Window
options" section unchecked?

Rick