Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Negative Influence
METABALLTAG_NEGATIVE
Parameter: Strength
METABALLTAG_STRENGTH
Parameter: Radius
METABALLTAG_RADIUS
Parameter: Type
METABALLTAG_TYPE
Parameter: Solid
METABALLTAG_SOLID