Hi ,
Instead of merely saying that something does not work , upload your workbook which has the data that you wish to format , so that others can see what the problem is.
The first link posted by chirayu clearly shows the format to be used ; a format string has two parts for numeric values , one for positive values , and another for negative values.
Enter the complete format string , and everything should work correctly , both for positive values and for negative values.
Narayan