When you first open the editor page, you’ll see a display like this:
The red circle shows the button you use to create a new encounter. It has a “+” sign. The “-“ button is used later to delete encounters. The encounters you build will be shown in the long column on the left.
When you press that “+” button, you’ll see the dialogue prompt in the center asking for the name you want for the new encounter. Simply type in the name and click on “OK”.