Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Radius
PRIM_NSIDE_RADIUSParameter: Sides
PRIM_NSIDE_SIDESParameter: Rounding
PRIM_NSIDE_ROUNDINGParameter: Radius
PRIM_NSIDE_RRADIUS