MATLAB Grader was designed to automatically grade MATLAB code and not Simulink models interactively. While Simulink models can be accessed through the use of the "sim" command in MATLAB Grader, those models can not be edited or accessed interactively through the GUI. Also, MATLAB Grader has a strict execution time limit of 60 seconds (which includes running the reference solution, learner solution, and all assessment tests). Due to the way that the "sim" command is currently supported, complex models, or models with long runtimes are not supported.
If you are willing to provide feedback on how you would like to incorporate Simulink models into automated assessments in MATLAB Grader, please contact your account representative.