To enter scoring criteria for your test project, click the Test Design button and then select the Grading tab:

In this example, we give grade A for the scores ranging from 91 to 100, grade B from 71 to 90, grade C from 61 to 70, grade D from 41 to 60, and grade E from 0 to 40.

Remember saving upon leaving whenever any modification has been made in Test Design.