A sequence contains all the content that's on the timeline - video and audio with their transitions, effects, etc. Usually there's only one sequence.
The length of a projects depend upon the amount of content. Still image durations can be adjusted simply by dragging their handles (on the left and right borders).

Audio/video clip length is what it is, but speed can be adjusted. Right-click on one to see that option.
Place audio/video clips on the timeline in the desired order. Click ADD CLIPS and locate your music file.
To see it in the bin (top-left of the VP window) clip the bin Audio Files tab. 
From there the clip can be dragged to the timeline beneath exiting audio tracks. Or move the red cursor in the timeline to where you want the clip to begin. Right-click the bin clip and OVERLAY ON SEQUENCE AT CURSOR. Overall music volume can be adjusted with the slider to the left of the track.

Recording narration automatically adds clips to the timeline in the same way. They are also saved under the Audio Files tab in the bin. If VP doesn't auto-place them on a track, do it manually, as above. The clips can be dragged right or left on the timelime, for accurate placement.
Check out the many VP tutorials, and drop by again if you need help. http://nch.invisionzone.com/topic/23872-videopad-tutorials/