Preassignments are added to a schedule when the "Create" button is clicked to make a new schedule. If you want to add preassignments to a schedule after it has already been created, you have two options:
Add the preassignment directly to the schedule.
This can be done by adding a volunteer to the first instance of the preassignment and then clicking the "repeats" bubble to create a new preassignment. This will then add the preassignment to the Preassignments pane, as well.
Add/edit the preassignments pane with the schedule open in the background.
If you have already added a preassignment to the Preassignments pane and you want to add it to the schedule, you can open the schedule and then minimize it. Go to the Preassignments pane and edit the preassignment. Make no changes and click to save it. It will now prompt you to add it to the open schedule. This is especially helpful when you have multiple schedules already live and need to add the preassignment to each of them.