Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Material Type
GV_MATERIAL_TYPE
Parameter: Exclude
GV_MATERIAL_EXCLUDE_ID
Parameter: First Material
GV_MATERIAL_INPUT_START
Parameter: Maximum Materials
GV_MATERIAL_INPUT_COUNT
Parameter: On
GV_MATERIAL_OPERATOR_ON
Parameter: Count
GV_MATERIAL_OUTPUT_COUNT