You can add multimeda content respectively interactive elements to your magazine in the so called "Hotspot Editor".
To get there go to "My magazines".
Then click on "Edit pages".
You are now in the hotspot editor. On the right side you can choose the page you want to add the element to (1). Then you can choose what element you want to add on the left side (2). To add an element click the icon on the left side and then drag the element open on the page (3). After that you can see a box on the right side in which you can configure the element (4).
Learn how to create video hotspots: here
Learn how to create audio hotspots here
Learn how to create slideshow hotspots here
Learn how to create navigational hotspots here
Learn how to export your hotspots here