When you first setup your MuseClass account, you will be prompted to create a Grading Calendar. You will be unable to use the functions of MuseClass until this step is completed.
Any member with an Admin account in your organization can navigate to the Admin tab and then go to the "Calendars" sub-tab to create a new grading calendar.
After selecting the "Create New Calendar" button in the upper right, complete the required fields and add as many grading periods as necessary.
Hit "Create Calendar" to finalize.