Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Index
GV_WEIGHTMAP_INPUT_INDEXParameter: Value
GV_WEIGHTMAP_INPUT_VALUEParameter: Value
GV_WEIGHTMAP_OUTPUT_VALUEParameter: Count
GV_WEIGHTMAP_OUTPUT_COUNT