Since you gave the VBA code im going to assume that it can be done this way:
Excel.Application oXL;
Excel._Workbook oWB;
Excel._Worksheet oSheet;
Excel.Range oRng;
try
{
oXL = new Excel.Application();
oXL.Visible = true;
oWB = (Excel._Workbook)(oXL.Workbooks.Add( Missing.Value ));
oSheet = (Excel._Worksheet)oWB.ActiveSheet;
oSheet.PivotTables("PivotTableName").PivotFields("PivotFieldName").ShowDetail = False
/pre>