When you press the Enter key in an essay question, it generally adds a line break, so you can start another paragraph. But you may find that the Enter key doesn't work in essay questions if you're viewing HTML5 output.
This issue was corrected in Update 3 for Storyline. To take advantage of the new features and fixes, download and install the latest version of Storyline from the link in your product confirmation email, or get it here. Then republish your Storyline project.