Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See page_maxonapi_getting_started_foundations_cinema_parameters.
Parameter: Radius
PRIM_NSIDE_RADIUS
Parameter: Sides
PRIM_NSIDE_SIDES
Parameter: Rounding
PRIM_NSIDE_ROUNDING
Parameter: Radius
PRIM_NSIDE_RRADIUS