Parameters are accessed with C4DAtom::GetParameter()/ C4DAtom::SetParameter(). See Parameter IDs.
Parameter: Count
MG_RADIAL_COUNT
Parameter: Radius
MG_RADIAL_RADIUS
Parameter: Plane
MG_RADIAL_AXIS
Parameter: Align
MG_RADIAL_ORIENT
Parameter: Start Angle
MG_RADIAL_STARTANGLE
Parameter: End Angle
MG_RADIAL_ENDANGLE
Parameter: Offset
MG_RADIAL_OFFSET
Parameter: Offset Variation
MG_RADIAL_RANDOM
Parameter: Offset Seed
MG_RADIAL_SEED