Oh sorry. That error you're getting is the same one you'll get with 6.1.15, so I jumped to this conclusion.
In the VS Tools | Extensions and Updates window, can you double-check the "DevForce 2010 EDM Designer Extension" to see that it's enabled and also its version number?
What is the target framework for the model project?
Do you also have the DevForce 2012 EDM extension installed?