Where the range is multiple cells:
Excel.Worksheet sheet = workbook.ActiveSheet;
Excel.Range rng = (Excel.Range) sheet.get_Range(sheet.Cells[1, 1], sheet.Cells[3,3]);
Where range is one cell:
Excel.Worksheet sheet = workbook.ActiveSheet;
Excel.Range rng = (Excel.Range) sheet.Cells[1, 1];