using Infragistics.Documents.Excel;
...
Workbook workbook1 = new Workbook();
Worksheet worksheet = workbook1.Worksheets.Add("Sheet1");
// Make some column headers
worksheet.Rows[1].Cells[1].Value = "Morning";
worksheet.Rows[1].Cells[2].Value = "Afternoon";
worksheet.Rows[1].Cells[3].Value = "Evening";
// Create a merged region from column 1 to column 3 that will be a header to the column headers
WorksheetMergedCellsRegion mergedRegion1 = worksheet.MergedCellsRegions.Add(0, 1, 0, 3);
// Set the value of the merged region
mergedRegion1.Value = "Day 1";
// Set the cell alignment of the middle cell in the merged region.
// Since a cell and its merged region shared a cell format, this will
// ultimately set the format of the merged region
worksheet.Rows[0].Cells[2].CellFormat.Alignment = HorizontalCellAlignment.Center;