If you have Tools|Options|Edit tab|Edit directly in cell
turned on, then you'll be editing within the cell--not in the formula bar.
(xl2003 menu system)
While doing some task i have come across one more way to do this.
Tools/Options/View tab/ check the formula bar check box and Ok.
By doing this the formula bar will appear below the menu bar you can either
put the formula directly or just click the fx sign and the wizard will open.