Look at the sidebar on the left of your Editor, where you can overview the deck and control the slides. At the bottom of these thumbnails, you'll find a "New Slide" button to add new slides, or you can go to "Insert" in the upper settings menu.
When adding a New Slide, Slidebean will give you some preset combinations to choose from and make things easier, or you can also insert a blank slide.
Scrolling down in this window, you'll also find the option to Import slides from another of your presentations.
Reordering slides within the deck is as simple as dragging and dropping them on the left sidebar.
To Delete a slide, click the little arrow next to its thumbnail and find the Delete option. Here you can also Duplicate the slide or add new ones.