private void button1_Click(object sender, System.EventArgs e)
{
//Get the GraphicsContainer
IGraphicsContainer graphicsContainer = axPageLayoutControl1.GraphicsContainer;

//Get the MapFrame
IMapFrame mapFrame = (IMapFrame) graphicsContainer.FindFrame(axPageLayoutControl1.ActiveView.FocusMap);
if (mapFrame == null) return;

//Create a legend
UID uID = new UIDClass();
uID.Value = "esriCarto.Legend";

//Create a MapSurroundFrame from the MapFrame
IMapSurroundFrame mapSurroundFrame = mapFrame.CreateSurroundFrame(uID, null);
if (mapSurroundFrame == null) return;
if (mapSurroundFrame.MapSurround == null) return;
//Set the name
mapSurroundFrame.MapSurround.Name = "Legend";

//Envelope for the legend
IEnvelope envelope = new EnvelopeClass();
envelope.PutCoords(1, 1, 3.4, 2.4);

//Set the geometry of the MapSurroundFrame
IElement element = (IElement) mapSurroundFrame;
element.Geometry = envelope;

//Add the legend to the PageLayout
axPageLayoutControl1.AddElement(element, Type.Missing, Type.Missing, "Legend", 0);

//Refresh the PageLayoutControl
axPageLayoutControl1.ActiveView.PartialRefresh(esriViewDrawPhase.esriViewGraphics, null, null);

//disable/enable buttons
cmdDraw.Enabled = false;
cmdDelete.Enabled = true;
cmdChangeArea.Enabled = true;
cmdChangeLine.Enabled = true;
}

private void button2_Click(object sender, System.EventArgs e)
{
//Find the legend
IElement element = axPageLayoutControl1.FindElementByName("Legend", 1);

if(element != null)
{
//Delete the legend
IGraphicsContainer graphicsContainer = axPageLayoutControl1.GraphicsContainer;
graphicsContainer.DeleteElement(element);
//Refresh the display
axPageLayoutControl1.ActiveView.PartialRefresh(esriViewDrawPhase.esriViewGraphics, null, null);

//enable/disable buttons
cmdDraw.Enabled = true;
cmdDelete.Enabled = false;
cmdChangeArea.Enabled = false;
cmdChangeLine.Enabled = false;
}
}