Hi sdnd2000,
1. please check if the namespace is defined correctly as below
xmlns:ds="clr-namespace:RIAExampleApp.Web"
2. After creating service, "Build"/"Rebuild" whole application is needed in order to access domain service on client side.
3. Right click on silverlight project (not .web project) - Properties - Silverlight - WCF Ria service link. here select the web part project.
Best Regards,