Here’s a C# example thats adds the circle as a graphic element on the mapcontrol.

//C#private void axMapControl1_OnMouseDown(object sender, IMapControlEvents2_OnMouseDownEvent e){if (e.button == 2){IRubberBand rubberband = new RubberCircleClass();IGeometry geometry = rubberband.TrackNew(this.axMapControl1.ActiveView.ScreenDisplay, null);CreateCircleElement(geometry, this.axMapControl1.Map);this.axMapControl1.ActiveView.PartialRefresh(esriViewDrawPhase.esriViewGraphics, null, null);}}private void CreateCircleElement(IGeometry geometry, IMap map){ISegment segment = geometry as ISegment;ISegmentCollection polygon = new PolygonClass();object Missing = Type.Missing;polygon.AddSegment(segment, ref Missing, ref Missing);IElement element = new CircleElement();element.Geometry = polygon as IGeometry;IGraphicsContainer graphicsContainer = map as IGraphicsContainer;graphicsContainer.AddElement(element, 0);}