While not a proper solution, a workaround has been posted by Microsoft: WCF Add Service Reference Not Supported for Windows Phone 8.1 XAML Applications
Windows Phone 8.1 XAML applications does not support the System.ServiceModel namespace, and therefore you are not able to right click references in the project and choose Add Service Reference. The recommended solution is to add a REST endpoint to your WCF endpoint, and access the WCF application through the REST endpoint using HttpClient.