This feature is available in Catalyst, Enterprise, and Ultimate editions.
Follow these steps to learn how to add a story/backlog item to a sprint/iteration.
Go to Sprint Planning (or Iteration Planning), and then click on the Sprint Scheduling (or Iteration Scheduling) tab.
Select a sprint/iteration on the left.
Scroll to the Backlog grid at the bottom of the page, select an item, and then drag into the sprint grid at the top of the page.
To schedule multiple items, select the checkbox next to each item, and then click Move to Sprint. When the Sprint Assignment window opens, select a sprint, and then click Move.
Expand any sprint container to view the list of backlog within it. After an item has been scheduled, it is removed from the Backlog grid at the bottom of the page. If the Sprint schedule does not already exist, you can create one as described in Creating a New Sprint Schedule.