Importing to stage or importing to library and dragging to stage should just work fine, so I am not sure what exactly is the problem. Can you share the file you are trying to import? And have you tried with new/empty FLA as well?
I am able to import it and view it on stage properly.
Can you follow exactly these steps and tell me if you still have the problem?
Open Flash CS5.
Select ActionScript 3.0
Press Ctrl+R and wait for file selection popup window to appear
Browse to the directory where the image is available
Select the image
If you have the file you are trying to import, open in any other program (The image you are trying to import) it will not place it on the stage. I just tested this from an image saved as a png from microsoft digital image pro (yet MDIPro still had the original on its desktop while its program was still running) however cs5 acted dumb as if there was nothing to place. I closed the file in the MIDIPro program and Voila! Flash CS5 recognized it and dropped it in on the stage.