Press Return (if necessary) to place the cursor on a new blank line. If the Scene Heading icon isn't highlighted, touch it to highlight it:
The grid will be displayed;
Touch the Scene Intro you want (INT., EXT., I/E), then touch the Location you want and finally the Time.
Just as in Final Draft, you can also use the keyboard to easily enter a Scene Heading: Type the first letter or two of the SmartType item you want and your choices will be displayed in the grid, then touch the Tab key to accept the sketched-in text.