<esri:Editor x:Key="MyEditor"
Map="{Binding ElementName= MyMap}"
LayerIDs="ThreatAreas" GeometryServiceUrl= "http://xxxxx/ArcGIS/rest/services/MapServices/Geometry/GeometryServer" />
You need to replace the highlighted texts with your map control's name, layer IDs (i.e.: "EditLayerClass1, EditLayerClass2"), and your own Geometry Service URL. Editor.LayerIDs is not required if all GraphicsLayers and editable FeatureLayers in your map will use the same Editor. Setting Editor.Map property should be sufficient in this case. Editor.LayerIDs is often used to specify which layers Editor need to work on.Note also that Add button will be disabled if its Editor contains more than one layer. In this sample, the same Editor is used because it was working against one layer only. You will need to update the Add button. Where you define another Editor that has only one LayerID and CommandParameter must match a feature type in your service.
<Button x:Name="AddButton" Margin="2"
DataContext={StaticResource EditLayerClass1Editor}
ToolTipService.ToolTip="Fire"
Content="Add Polygon"
Command="{Binding Add}">
<Button.CommandParameter>
<sys:Int32>10301</sys:Int32>
</Button.CommandParameter>
</Button>
Alternatively, you can use TemplatePicker for Add. http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#ToolkitTemplatePicker