How did you import your video? You need to import as Multi-track Project to get into the project for adding music. Importing as file will open the waveform for adjusting/correcting that one audio file, not for mixing/composing.
The workflow would be open a new project, use the STP browser to find your video, and drag it into either the video window (above the browser) or drag it onto the timeline.
Hope this helps.
Cameron Young