Hi
You may be mistaken in two ways....... 
"The Overlay sequence line will contain a grey box with some of your text showing in it."
(1)
The overlay sequence line mentioned here is the line directly below your video sequence track, and above the audio track......NOT the light gray box at the left hand end with the green cross. As you have found, clicking this will have no effect.
If you have added some text overlay (like titles etc.)to your project, it will appear in a dark grey box underneath the video track but above the audio track and some of the text (depending its length) will be visible. This is the box that I am referring to in my posting.
(2)
Now, as you say you are working on a file that has already been saved as a movie. This will NOT show overlays. VP (and any other editor) will see this as single,complete clip.
On loading, it will appear only as a video on the sequence line with the associated sound on the audio track. Any titles/end credits/subtitles etc. will be part of the film and will not appear on the overlay track But the film is editable in the normal way just like any video clip.
Note: It is only when you are dealing with or have loaded a VP PROJECT that any text is seen as an overlay. As you edit your clips you normally save your work as a VP project...In essence this is just a series of instructions that tells the program how to recreate your work when you come back to it after a layoff. Now, once you save your project as a finished movie everything is incorporated (text/overlays/edits/transitions)and assembled and you can save it to DVD or your PC etc. Loading THIS back into VP (which is what you seem to have have done) is just the same as loading a new (but bigger) video clip. It IS editable but now the text is part of the film.
If the VP data file - the VP project for your movie is still available you should load this to VP and then you can edit the text (and anything else) to your hearts content.
Hope this helps
Nat