Sorry, I mean that you need to right click on a track in the map window -- not on the file in the active file list. And you should be in pan mode (hand cursor).
Area is definied per track, not per file, that's why it is in that drop down.
A problem with the interface right now is that some functions (like this one) are hidden in the right click menus. I guess it is not obvious that you can right click on tracks in the map window since some people seem to be missing it.