Thanks Tamlin for replying.
Well I did insert bg_mc for movie clip instance.
Anyway I did screen shots of my steps. Can you tell me what I did wrong?
1. Create new doc. 300 px x 200 px. 25 fps. Click ok.
http://img.photobucket.com/albums/v177/JJNEOGEO/s1.gif
2. Insert picture. I took it from Kirupa's.
http://img.photobucket.com/albums/v177/JJNEOGEO/s2.gif
3.Select picture and convert it into Movie Clip. I named it as cloudsMC. Click ok.
http://img.photobucket.com/albums/v177/JJNEOGEO/s3.gif
4. Click that picture again and insert bg_mc in Properties.
http://img.photobucket.com/albums/v177/JJNEOGEO/s3.gif
5. Insert script on the first frame of the timeline.
http://img.photobucket.com/albums/v177/JJNEOGEO/s5.gif
6. Not working.
Would very much appreciated if you can tell me what I did wrong.