If your video doesn't have audio in it, it will appear as a grey rectangle after uploading it into your project.
Timeline Mode
Double-click on the clip in your timeline (bottom middle of your screen should have thumbnail icons of the clips in your project). This will open up a new page - clip editor mode!
Add Design
Above the clip timeline, you'll see an option to 'Click to Add Design'.
Add as many designs as you'd like to your clip!
Video Tutorial