The format cells standard offering does not offer brackets around negative numbers. You have to create your own custom format.
So, under FORMAT -> CELLS, select CUSTOM and type in the following:
#,###.00_);(-#,###.00);0.00
What the above shows is:
#,###.00 <- the format for a positive number
(-#,###.00) <- the format for a negative number (if you don't want a minus sign in front of the negative number, then just remove it)
0.00 <- shows the format of all numbers, i.e. that there are 2 decimal points after the integer.