1. Enter Statistics & Grading
2. Switch to Statistics & Grading by Topic
3. Select a Topic
4. If the topic is not already associated with a GB item, select an item to associate.
You'll end up with a grading screen. To the right of the "Comments" column you'll find a small field with a "0" and the option to "Apply Grade to All Ungraded".
This should appear BELOW the table.