I had the same 'problem', so I played a bit with WMP and found the solution. Here it is:
If you want to be able to rate music files:
1. Switch to Library View 2. Right-click on Music(under Library and Playlists in the leftmost panel) 3. Choose Manage Music Library 4. Add the folder where you music files are 5. Restart and you are ready to rate!
Basically, it's the same procedure for videos, pictures and etc.(just right click on Videos/Pictures/Whatever expandable menu in the leftmost panel)
PS: you can right-click and customize the navigation pane to add Folder view(most convenient for me), Ratings view and many more.