I struggled with this one as well.

The instruction should say Right-Clic / Edit on the "US airplane Tracker" layer label in the "places" pane, not on the actual graphic layer (which is what I was initally trying to do). At least, that's how I go to it...

This will then bring the Edit Image Overlay window where you can add the flight number (UAL 201) to the URL alreay in there...

Hope this helps.

Great job (although the scaling can be tricky)!