Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Middle Point
MDATA_SPLINEARC_MIDPOINTParameter: End Point
MDATA_SPLINEARC_ENDPOINTParameter: First Point
MDATA_SPLINEARC_FIRSTPOINTParameter: Center
MDATA_SPLINEARC_CENTERParameter: Radius
MDATA_SPLINEARC_RADIUSParameter: Angle
MDATA_SPLINEARC_ANGLEParameter: First Insert Spline Point Index
MDATA_SPLINEARC_POINTINDEX1Parameter: Second Insert Spline Point Index
MDATA_SPLINEARC_POINTINDEX2Parameter: Object First Insert Spline Point Index Belongs To
MDATA_SPLINEARC_OBJECT1Parameter: Object Second Insert Spline Point Index Belongs To
MDATA_SPLINEARC_OBJECT2