Thanks, folks. I guess it was a bug from my original install before I re-formatted my hard drive. When I did it the first time I played, it showed up immediately after chapter 1 was complete, it was just always greyed out. Then, when chapter 3 began, it disappeared altogether. Then, during chapters 4 or 5 (I forget), something happened so that it returned to my map, but I still couldn't go there. I knew I must be missing something.