When you create a new unit, a new lesson will automatically be generated and relate to that unit. For any new lessons, you will have to connect the unit under the Related Curriculum property, otherwise when you open the unit page, you will not see the list of related lessons under that unit.
Notice that the curriculum database is grouped by course, any new unit or lesson added under that group will be automatically assigned the Course property. You may easily switch a unit/lesson from one course to another by dragging it into the appropriate course. (if you cannot find a page, it may be in a hidden group - change visibility under database settings, selecting group, and toggling hide empty groups)

Curriculum

Calendar