"Scrumy's great, but where's the product backlog?" -Certified ScrumMasters everywhere
In the Backlog, you create stories independent of sprints. Some people use a product backlog as a master list of stories, and then pull a selection of those stories into a new sprint. If you can think of another creative use for it, go for it!
There are three priority levels for backlog stories: Low, Medium, and High. To move stories to a lower or higher priority, simply drag them up or down. The idea here is that you drag stories lower and lower until they eventually end up on the live board. Certified ScrumMasters everywhere like to prioritize their stories to see what should be done next. I like to call this implementation The Technicolor Waterfall (sorry if the "waterfall" word still makes you shudder...).
In the real world, you're probably going to be moving more than one story at a time. To move multiple stories from the Backlog into a sprint, click them to select them, just like you already do with tasks in the Dashboard. From there you can move the stories with the blue "Move" buttons that will appear, or you can click the "New sprint" button and start a new sprint with the selected stories.
As usual, you can enter in time estimates into your stories if you'd like, via the same mechanism as usual. Once those stories make it into a sprint, the time estimates will be reflected in the Burndown Chart.
If you don't have a Pro account, you can always play with the latest Pro stuff in the demo account, http://scrumy.com/demo.
We hope you like it!