A Yearbook ID is a 5 to 6-digit number associated with your project.  These can be given out when calling for technical support!

To find it, simply look to the top right of the dashboard while not working in the designer.  You should see a yellow information bar, which also functions as a way to switch projects.  In the screenshot below, you should see “#56215”.  This is the yearbook ID for the project (yours will be different than the one shown).