There is a property of the CNiReport object called "Orientation". You just set that property to Landscape using the enumeration provided in the CNiReport class. Here is an example with the full path to the enumeration value:
m_report.SetOrientation(NI::CNiReport::NiReportsPageOrientation::NiReportsLandscape); //m_report is the CNiReport object
Best Regards,
Chris Matthews
National Instruments