All unscheduled tasks are displayed in the Unscheduled Work column on the right. A task is unscheduled if it does not have a Planned Date. Tasks can be scheduled by dragging and dropping into a month in the planner, which will set the Planned Date to the first day of the month.
The Unscheduled Work column can also be filtered to help you find a particular task. Just use the search bar at the top of the column and enter the name or client of the task you're looking for.
You can also schedule a task by editing it and assigning it a Planned Date.