SpreadsheetGear supports reading/writing/displaying workbooks using either of Excel's two major file formats--Excel 97-2003 (.xls) and Open XML (.xlsx or *.xlsm). See the SpreadsheetGear.FileFormat enum for more information.
Use the Factory.GetWorkbook() method to open a workbook from disk, then set your WorkbookView's ActiveWorkbook property to the IWorkbook object returned from Factory.GetWorkbook(...)
to display it. There is also a WorkbookView.ActiveWorksheet if you need to specify a particular worksheet within the workbook to display. Example:
IWorkbook workbook = Factory.GetWorkbook(@"c:\path\to\workbook.xlsx");
worbookView.ActiveWorkbook = workbook;