Since you have the center and radius, you can derive the points along the circumference and produce a polygon.
private ESRI.ArcGIS.Client.Geometry.Polygon DrawCircle(ESRI.ArcGIS.Client.Geometry.MapPoint center, double radius, int pointsCount = 360) {
ESRI.ArcGIS.Client.Geometry.Polygon p = new ESRI.ArcGIS.Client.Geometry.Polygon();
ESRI.ArcGIS.Client.Geometry.PointCollection pcol = new ESRI.ArcGIS.Client.Geometry.PointCollection();
double slice = 2 * Math.PI / pointsCount;
for (int i = 0; i <= pointsCount; i++) {
double rad = slice * i;
double px = center.X + radius * Math.Cos(rad);
double py = center.Y + radius * Math.Sin(rad);
pcol.Add(new ESRI.ArcGIS.Client.Geometry.MapPoint(px, py));
}
p.Rings.Add(pcol);
return p;
}