Tracking Changes in Excel 2007

1 Click on the “Review” tab on the Ribbon in Excel 2007
  review-tab
2 The “Changes” group is on the far right hand side of the Ribbon
  changes-group
3 Click on “Track Changes” and “Highlight Changes”
  Track-changes
4 Check “Track changes while editing.  This also shares your workbook.”
  highlight
5 Select what you want highlighted
  highlight-2
6 Click OK

(source: The New PaperClip)