Hi Beryl,
Thanks for contacting Syncfusion support.
By default the Silverlight assemblies present in the GAC which contains the assemblies for Silverlight4.0. The Assemblies for Silverlight5.0 is available under the following location, referring the assemblies will solve the error.
[System drive]\Program Files (x86)\Syncfusion\Essential Studio\10.X.X.X\Assemblies for Silverlight5\
Please let us know if you have any queries.
Regards,
Karthikeyan