The Storyboard Editor (SBE) is located in the Design Tab (keyboard shortcut F2) in the Beatmap Editor. Fundamentally, the SBE offers users an easy way to control the movement of sprites, but is often regarded as clunky and unusable for anything beyond the basics of storyboarding. If you wish to create a widescreen storyboard, you can do so by going to the song setup dialog and checking the Widescreen Support box in the Design tab.
Users skilled in storyboard creation often use the backend code created by the SBE, entitled Storyboard Scripting (SBS) instead, as it offers them more control. See that article for details.