Only one essay is showing at first.
You see, one of the essays is tied to the school to which you apply. To get that essay to show up in the writing section, you need to answer the academic questions. Luckily, the Common App is smart enough to alert you to the fact that you aren't done if you try to just fill in one essay and submit it.
Start at the beginning and fill out your academic information.
When you work through the questions and get to the writing section, you'll see that school-specific question on the page.
Good luck!