Where is System.ServiceModel.DomainServices.EntityFramework DLL?

I'm trying to use RIA services, I installed Silverlight 4 and Silverlight 4 Tools/SDK, but I cannot setup a LINQToEntitiesDomainService because I can't find this namespace/assembly: System.ServiceModel.DomainServices.EntityFramework. Where does this assembly come from, not sure why it's missing? Is that an add-on I'm missing?

Thanks.