To use the Electronic Index Cards, you need to enter Index Card View:
Under the View menu, select Index Cards.
Your document now looks something like the following, depending on the number of cards or scenes you've created:
Each card represents one scene. Across the top of each card are two boxes: the leftmost box contains the Scene Number, while the rightmost box contains the Scene Heading. Beneath is the scene text, comprised of Action (and Character Names and Dialogue if you've entered them in your document.)