See the image below, please help me, thanks.
Windows 7's Explorer doesn't list sub-folders there. I don't think Vista's did either, from memory.
That aside, it's not something you can change in Opus.

Why do you want to navigate your filesystem using a series of little popup menus when you can just use Opus itself?
Of course, the navigation in Opus is just great, but I want to see the list of the sub-folders in the selected folder as well. I don't often use folder tree, the path completion in path fields is good but sometimes I even can't exactly remember the name of the folder to navigate to, that is the problem.
Well, you can consider this as an issue to perfect more our DOpus, that's all. Thanks.