Hello Francisca,
As shown on the video Part 3, stories are created in the Sandbox, not in the Backlog. Once created in the Sandbox they can be then accepted to the Backlog.
Then, Backlog stories must be estimated as shown in Part 4 so they can be planned into Sprints, for instance in the Release Plan view as shown in Part 5.
To sum up the beginning of the story workflow:
- Sandbox: Create
- Sandbox: Accept to backlog
- Backlog: Prioritize (optional)
- Backlog: Estimate
- Release Plan: Plan story from backlog widget to sprint
An estimated story can also be planned into a sprint in is « update » form. This is the regular workflow from the free version, there is a slight difference in iceScrum Pro where you an alternatively create stories directly in the Backlog, skipping the Accept step.