I tried to reproduce the problem in my local tests by adding Intersoft Entity Data Model but my efforts were not successful -- everything worked smoothly and the model designer was accessed without any issues.

For your information, I'm using the latest version of Mobile Studio setup file.

From the error message, there might be something wrong with the Intersoft.Data.CodeGenerator assembly. I'd like to suggest you to apply the nightly build of Crosslight 2 which available in here.

After download the nightly build, please carefully follow the step-by-step guidance in applying the update that can be found in here.
Last, copy Intersoft.Data.CodeGenerator.dll from "C:\Program Files (x86)\Intersoft Solutions\Crosslight 2\bin\Intersoft.Data.CodeGenerator.dll"; and replace the assembly in "C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\IDE\PublicAssemblies\Intersoft.Data.CodeGenerator.dll" withh the new one.

Please note that you need to ensure to close all Visual Studio instance during the process. If the problem still persist, could you please send us a copy of your model file to be investigated further.