I'm not sure, either, what you're saying, isla, but in case jsuarezg is barking up the wrong tree here's my own theory on what's going on: You're used to seeing a cell's formula being edited up at the top in the formula bar, but now Excel thinks it's supposed to display it in the cell itself. Or vice versa. Either way, you're looking for the editing as it happens, and not seeing it because you're looking in the wrong place.

This is controled in Excel's options: Select File, then Options, then click on Advanced at the left. The first set of options under Advanced is headed "Editing options" in bold; look in that section until you see "Allow editing directly in cells", and check or uncheck that option to suit you.