Recently, I've been working with Windows Workflow Foundation 4 a lot. I wanted to use a FreeformActivityDesigner for something but the System.Workflow.* assemblies were not listed in the Add Reference dialog. The problem was that my Solution was targeting the .NET Framework 4 Client Profile Platform Update 1 which doesn't contain those assemblies. To fix this problem just change the target to .NET Framework 4 Platform Update 1.
Edit: It looks like the FreeformActivityDesigner is not really usable in WWF 4 and that the alternative is WorkflowItemPresenter. (forum post)