In order to create a rubric for a discussion board, follow the below steps:
1. Browse to the discussion forum that you wish to attach the Rubric to.
2. In the Forum Settings (see How do I create a discussion board? for more on accessing the settings) select Grade Discussion Forum
3. This will generate new rows called Due Date and Associated Rubrics. Click Add Rubric.
4. Next choose Create New Rubric.
5. All fields in the Rubric are editable and rows and columns may be added or removed: