F5 just displays the 'Goto' box, with no radio buttons or 'blanks'. And what though have you given to the effects this has on formulae in the destination range?

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

Macropod, you're correct, I did skip a step in my explanation. it should have said:
type a zero into a cell, copy it to your clip board, highlight the cells you want to replace blanks with zeros in, hit F5, Click the "Special" button, then click the radio button next to "blanks"; click OK, then hit CTRL+V

As for formulae in the destination range, if the cell is blank, it wouldn't necessarily have a formula in it. Perhaps my solution isn't the appropriate solution for the OP's scenario in that regard. It was intended to be a helpful suggestion.