Hello,
All configuring for coded-value domain is done prior to consuming the service in the ArcGIS Viewer for Silverlight. You may need to verify the settings on your layer/published service. For comparison, ssee the following service where the Facilities layer has coded-value domains and the Label appears in pop-ups and the attribute table instead of the domain value: http://sampleserver5a.arcgisonline.com/arcgis/rest/services/LocalGovernment/Recreation/FeatureServer...
Katy