How to Add a Comment to a Cell in Excel 2013
Click the New Comment command button on the Ribbon’s Review tab or press Alt+RC.
A new text box appears. This text box contains the name of the user as it appears in the User Name text box on the General tab in the Excel Options dialog box (Alt+FT) and the insertion point located at the beginning of a new line right below the user name.
Type the text of your comment in the text box that appears.
These comments serve as reminders, so enter the pertinent information.
When you finish entering the comment text, click somewhere on the worksheet outside of the text box.
Excel marks the location of a comment in a cell by adding a tiny triangle in the upper-right corner of the cell. (This triangular indicator appears in red on a color monitor.)