Once you have navigated to the Binder section of your event, select "Add your first folder" to create your first folder.
You can name your folder, add a description, and select if you want to make it public or private. A public folder is great for files you want your attendees to have access to.
When you return to your Binder later, select the "Show" button next to the folder you wish to open.
From there you can click "Choose Files" to add more files to the folder.