in Ecxel 2004, from help (it is useful sometimes!):
1. Click (once) on the data series to which you want to add error
bars.
2. On the Format menu, click Selected Data Series.
3. On the X Error Bars tab or the Y Error Bars tab, select the options
you want.
if you have you own calculated error values you will need to choose
the "custom" button and then define the reference to the apprpriate
area in your data table. Note that under this option excel allows for
asymetric errors and you have to enter values for both positive and
negative errors.