Adding a comment

You can add Comments (in later Excel versions called Notes) either via the ExcelRange class or via the Comments property on the ExcelWorksheet class.
Comments/Notes can be added using plain text or using richtext as descibed in this example below.

using(var package = new ExcelPackage(@"c:\temp\commentstest.xlsx"))
{
    var ws = package.Workbook.Worksheets.Add("comments");
    //Add Comments using the range class
    var comment = ws.Cells["A3"].AddComment("Jan Källman:\r\n", "JK");
    comment.Font.Bold = true;
    var rt = comment.RichText.Add("Here is some text that isn't bold...");
    rt.Bold = false;
    rt.FontName = "Helvetica";
    rt.Color = Color.Purple;
    comment.AutoFit = true;

    //Add a comment using the worksheet's Comments collection
    var comment2 = ws.Comments.Add(ws.Cells["B3"],"Jan Källman:", "JK");

    //This sets the size and position. (The position is only when the comment is visible)
    comment2.From.Column = 7;
    comment2.From.Row = 3;
    comment2.To.Column = 16;
    comment2.To.Row = 8;
    comment2.BackgroundColor = Color.White;
    comment2.RichText.Add("\r\nThis comment has white background and size/position set...\r\n");
}