Check the version number on of the following assemblies in GAC:
System.Workflow.ComponentModel.dll
System.Workflow.Activities.dll
System.Workflow.Runtime.dll
Microsoft.Workflow.VSDesigner.dll
They all need to match. If you don’t have the VSDesigner dll then you don’t have the VS Extensions installed, which would give you the error you are getting, and if the first three are missing you don’t have the runtime components installed.