First of all, make sure that:
<Tools> <Options> <View> tab,
"Zero Values" *IS* checked.
Next, do the empty cells contain formulas, or are they *truly* empty.
If they're truly empty, in an unused cell, key in a 1.
Then, right click on this cell and choose "Copy".
Select all the cells in question, and right click in the selection.
Choose "Paste Special", click on "Multiply", then <OK>.
And you should have all your zeroes.
If your empty cells contain formulas, post back with an example.