Where is System.ServiceModel.Web.dll?

Apparently when you select .NET framework 4.0 while creating the project, Visual Studio 2010 Beta 2 actually targets .NET framework 4 client profile. Change the target framework to .NET framework 4 using project properties.

Please look at the following folder for System.ServiceModel.Web.dll

C:\Program Files\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\

Are you targetting .NET4 Full or Client Profile? The latter does not include the System.ServiceModel.Web assembly.