In a recent project I tried to use the UIScrollBar component of Flash 8. There are already a lot of tutorials on the web:
There are also people writing comments on the Adobe livedocs. Such things cost me at least one hour to find what happens.
The key to the solution is: you must enter something long enough in the text box before loading anything into it! I guess the UIScrollBar component just caculates the behavior before it's published. The only size it knows is your text size in the design phase.
So, don't forget to paste some dummy text in your text box and give it an instance name before you use the UIScrollBar component!