To view and grade assignments stored in a Dropbox (individual or collective), click on Evaluation > assignment.
Assignments list
In the list, click on the assignment to be graded
Click on "Go to evaluation" next to the group/student you wish to assess.
Grade
Enter the note and comment.
Click on "Publish" if you wish to publish the note immediately (it will be visible to students).
Click on "Save Draft" if you don't want to publish the note right away (you can do so later en masse, see next step).
Publish the grades
Select all the assignments for which you wish to publish a grade (to select them all at once, click on the box in the top left-hand corner, making sure that all assignments are displayed on the same page).
Click on "Publish Feedback".