First, you need to be logged in to add a session. On the event schedule page you will find a "Plan session" button in the top-right corner. This will start a wizard that help you to plan a session. Alternatively, on desktop you can click in an empty slot to add a 60 minute session with that start time and location. Adding a session requires at least a location, time and title, but it's highly recommended to enter a summary as well.