Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Input
GV_VECT2REAL_INPUTParameter: X
GV_VECT2REAL_OUTPUTXParameter: Y
GV_VECT2REAL_OUTPUTYParameter: Z
GV_VECT2REAL_OUTPUTZ