Hexawise "Notes" allow you to add comments and/or questions to plans
Notes are specific to individual plans. They are located under Review in the left navigation pane.
Fill in the details of your note (1) and click the "Create" button (2)
Keep in mind that anyone with access the plan (including reviewers, etc.) can use this feature. This is usually a good thing.
How many notes have been created for a plan? That information is shown here:
Click on the trashcan icon (appears on hover) to delete an existing note
Click "Yes" in the confirmation dialog to confirm the deletion.
Make sure to take good notes when designing your test plans! You'd be surprised how much you forget by the time the next release comes around.