Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: X
GV_REAL2VECT_INPUT_XParameter: Y
GV_REAL2VECT_INPUT_YParameter: Z
GV_REAL2VECT_INPUT_ZParameter: Output
GV_REAL2VECT_OUTPUT_VECTOR