When you create a Windows Communication Foundation (WCF) project in Microsoft Visual Studio 2010, the Microsoft.SharePoint.Client.ServerRunTime.dll reference is not available on the .NET tab. Therefore, you cannot add this reference.
To resolve this issue, locate the GAC_MSIL directory, and from there, manually add the reference to Microsoft.SharePoint.Client.ServerRunTime.dll. GAC_MSIL is located in the root GAC directory:
Article ID: 983076 - Last Review: Oct 19, 2010 - Revision: 1