How to display zero value as blank cell in mac numbers 3.1
The easiest way is to use "Conditional Highlighting..." found under the "Format" menu.

Select the cell you want to have blank as zero then select "Conditional Highlighting" you then set a highlight when the cell is equal to zero. Choose  "Custom Highlighting" at the bottom of the style list and then you can set the text colour to white. A "blank" cell on zero.