NO SE ENCUENTRA EL System.ServiceModel.Web CUANDO CREO UN SERVICIO REST SOLUCIÓN

1. right click on the project name and choose Properties.
2. change the target framework to .NET Framework 4.
3. right click on the References and choose Add Reference.
4. And then you can see System.ServiceModel.Web.

By default the target framework is .NET Framework 4 Client Profile, so you cannot find the System.ServiceModel.Web.

Referencia:
http://stackoverflow.com/questions/1039839/wcf-cannot-resolve-webget-symbol-what-am-i-doing-wrong