You didn't say where you are getting the ExcelHPageBreaksHandle handle for the HPageBreaks collection. What you would have to do is get the HPageBreaks collection from the WorksheetObject, then Add a page break to it. Somthing like this:
CA_VariantSetCString (&MyCellRangeV,"A56");
Excel_WorksheetRange(ExcelWorksheetHandle, NULL,MyCellRangeV,CA_DEFAULT_VAL, &ExcelRangeHandle);
Excel_GetProperty (ExcelWorksheetHandle, NULL, Excel_WorksheetHPageBreaks, CAVT_OBJHANDLE, ExcelHPageBreaksHandle);
Excel_HPageBreaksAdd(ExcelHPageBreaksHandle,NULL,ExcelRangeHandle, &ExcelHPageBreakHandle);
This would add a horizontal page break before the "A56".
Best Regards,
Chris Matthews
Measurement Studio Support Manager