Atleast when working with mpk's this is how you can do it: (snipped from old test, not guaranteed 😃
<Window.Resources>
<esri:LocalFeatureService x:Key="FeatureService" Path="Data\\stuff.mpk" MaxRecords="3000" EnableDynamicLayers="True"/>
<esri:SimpleRenderer x:Key="MySimpleRenderer">
<esri:SimpleRenderer.Symbol>
<esri:SimpleFillSymbol BorderBrush="Blue" BorderThickness="4" Fill="Red"/>
</esri:SimpleRenderer.Symbol>
</esri:SimpleRenderer>
</Window.Resources>
<Grid>
<esri:Map x:Name="_map" UseAcceleratedDisplay="True" IsEnabled="false" >
<esri:ArcGISTiledMapServiceLayer ID="World Topo Map"
Url="http://services.arcgisonline.com/ArcGIS/rest/services/World_Topo_Map/MapServer"/>
<esri:ArcGISLocalDynamicMapServiceLayer Service="{StaticResource FeatureService}"/>
</esri:Map>
</Grid>
</Window>
or from code:
private const string PATH = "Data\\stuff.mpk";
private LocalMapService _mapService;
LocalMapService.GetServiceAsync(PATH, localMapService =>
{
_mapService = localMapService;
_map.IsEnabled = true;
});