Your inbox is where Mindjoy tells you what has happened: new work to do, reminders when something is due soon, and work your teacher has returned.
Opening it
Click Inbox in your sidebar. The number is how many you have not read.
Each notification has a button that takes you straight to the lesson or assignment it is about.
What you'll see
New work. "Added [lesson] to [class]", with when it starts and when it is due.
Due soon. If you have not finished something, Mindjoy reminds you as the due date gets close, for example "[lesson] ([class]) is due tomorrow". Once you submit, the reminders stop.
Returned work. When your teacher returns a lesson, you get "Returned [lesson] ([class])" with a Review feedback button. That is the one to look for: your marks and feedback are ready.
Some lessons give you feedback the moment you answer or submit. Those do not send a returned notification, because you have already seen your feedback on the lesson.
Getting them by email
Click your school's name at the bottom of the sidebar and choose Settings, then Email preferences. Tick Send me a daily digest email of my notifications and click Save. You will get one email a day listing anything you have not read yet.