mandikor
Click Format Cells and then select "Number" from the Category list and you should see the brackets option.
However, most annoyingly, I don't think Microsoft have yet woken up to, or fixed, the fundamental problem with this format - the digits of values in columns don't line up when there are negatives, as the closing bracket uses up the right hand character position.
To overcome this you must create a custom number format (select the number format most like the one you want, and then select Custom from the Category list, then edit the format in the Type box) so it looks like this:
#,##0_);[RED](#,##0);"-"_)
The digits will then line up, negatives will have brackets and be coloured red, and zero values will display as "-" [minus the "]
Also check out Creating Custom Formats in Excel's Help.
HTH
BigC