Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Negative Influence
METABALLTAG_NEGATIVEParameter: Strength
METABALLTAG_STRENGTHParameter: Radius
METABALLTAG_RADIUSParameter: Type
METABALLTAG_TYPEParameter: Solid
METABALLTAG_SOLID