Komal,
We have an online sample to add a marker on the server side which you can access at http://websamples.thinkgeo.com/webeditionsamples/; please have a try.
Here is the code to add markers on the client side. Hope it helps.
var OnMapCreated = function(map) {
var markers = new OpenLayers.Layer.Markers("Markers");
map.addLayer(markers);
map.events.register("click", map, function(evt) {
var pixelxy = new OpenLayers.Pixel(evt.x, evt.y);
var lonlat = map.getLonLatFromPixel(pixelxy);
var size = new OpenLayers.Size(20, 20);
var offset = new OpenLayers.Pixel(-(size.w / 2), -size.h);
var icon = new OpenLayers.Icon('/theme/default/img/marker.gif', size, offset);
var markers = map.getLayersByName('Markers')[0];
markers.addMarker(new OpenLayers.Marker(lonlat, icon));
}
);
}
If you have any more questions, please feel free to let me know.
Thanks,
Howard