Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Matrix
GV_MATRIX2VECT_INPUTParameter: Offset
GV_MATRIX2VECT_OUTPUT0Parameter: V1
GV_MATRIX2VECT_OUTPUT1Parameter: V2
GV_MATRIX2VECT_OUTPUT2Parameter: V3
GV_MATRIX2VECT_OUTPUT3