Well if you already have the polyline then you have some of this done already. this is an example of what it would take to build a polyline from nothing and add it to the map.
// What my symbol looks like
Symbols.SimpleLineSymbol symbol = new ESRI.ArcGIS.Client.Symbols.SimpleLineSymbol()
{
Color = new System.Windows.Media.SolidColorBrush(System.Windows.Media.Colors.Blue),
Style = Symbols.SimpleLineSymbol.LineStyle.Solid,
};
// the points that make up my shape
PointCollection pc = new PointCollection()
{
new MapPoint(0,0, new SpatialReference(4326)), // first point in line
new MapPoint(1,1, new SpatialReference(4326)), // second point in line
};
Polyline polyline = new Polyline()
{
// don't for get to let the map know what spatial reference this is
SpatialReference = new SpatialReference(4326)
};
polyline.Paths.Add(pc);
Graphic g = new Graphic()
{
Symbol = symbol, // add my symbol to my graphic
Geometry = polyline // add my geometry to my graphic
};
GraphicsLayer graphicLayer = new GraphicsLayer();
graphicLayer.Graphics.Add(g);