You'll have to use a custom format for this. If you refer to Excel help and search for "Create a custom format", you'll see that you can customise a format, using four sections, separated by semicolons.
The first section deals with postive numbers, the second for negative (this is the one you want to edit) etc etc.
So, if you select one of the built-in custom formats similar to this one:
_-* #,##0.00_-;-* #,##0.00_-;_-* "-"??_-;_-@_-
.....you can edit it so that the second section contains your brackets, to something like this:
_-* #,##0.00_-;(-* #,##0.00_-);_-* "-"??_-;_-@_-
........ but use the Excel help for the exact meanings of the various formats.