If you wish the results to be used in another cell, than this can be done using a formula:

= "(" & cellref & ")"

where cellref is the cell where the desired text is located (e.g., B3).

A custom format can also be applied directly to a cell which can display the content in parentheses:

1. With the desired cell selected, click "Format - Cells"
2. On the "Number" tab, select the category: "Custom"
3. For the Type, enter: (@)
4. Click OK.

(see Conditional Formatting at Excel Tutorials)