using Excel = Microsoft.Office.Interop.Excel;
...
object mis = Type.Missing;
Excel.FormatCondition cond =
(Excel.FormatCondition)range.FormatConditions.Add(Excel.XlFormatConditionType.xlCellValue,
Excel.XlFormatConditionOperator.xlEqual, "=1",
mis, mis, mis, mis, mis);
cond.Interior.PatternColorIndex = Excel.Constants.xlAutomatic;
cond.Interior.TintAndShade = 0;
cond.Interior.Color = ColorTranslator.ToWin32(Color.White);
cond.StopIfTrue = false;