Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Normal
GV_MATRIXMULVECT_NORMALParameter: Matrix
GV_MATRIXMULVECT_INPUT1Parameter: Vector
GV_MATRIXMULVECT_INPUT2Parameter: Vector
GV_MATRIXMULVECT_OUTPUT