Hi Ellen - I will try to assist. In looking at your gradebook screenshot the overall gradebook is set to Simple Weighted Mean of Grade - you need to change that to Weighted Mean of Grades - then scroll down and click save. Once Weighted Mean of Grades is selected then there will be a field where you can enter the grades. If when you pull down on the menu for Aggregation - and Weighted Mean of Grades is not there you will need to contact the Moodle Admin. The aggregation you need to set will be at the top of the gradebook where the title of the course appears. I hope this is of some help. You also might check on YouTube - I did a session at Moodle USA in 2015 about the gradebook - that be of some assistance.
