Cancel shouldn't be that hard, but at the moment you have to complete the scene start and end frame selection or transition selection before cancel will enable.

I am hoping that the "Add" button will not challenge anyone either but I guess it won't hurt to comment about it.  Add, simply put, adds the scene you have selected by source file and start / end frames, effects or transitions to your project time-line or scene list.  Your selection will show in the window as either a list or as boxes and thumbnails below a frame ruler.

Here you see the result of adding a scene and a dissolve to a project.  Note how the transition hangs below and overlaps the ending frames of the first element.  The next scene you add will overlap the transition completely leaving a transition tab hanging down.

So add a couple of more scenes.  Get the feel of it.