Have you let run the MxModel reflection? I think this is required to display the entities you can use.
I had to add the Mx Object snippet from the MxModelReflector Module onto a new page in my project and then add this page to the navigation menu. Once running the app I had to select the modules that i want to retrieve attributes from and refreshed the attribute list. Once I returned back to the administration page for the email I was able to select the attributes I wanted.