depending on which version you have. flash 5 i think and up to flash mx 2004 have scrolling components that you can drag onto your text boxes to make them scroll. Make sure the text box is set to dynamic and multiline.
btw. if you hit F11 itll open up your components window.
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In order to make your own (dynamic) scrolling area in Flash, you'll have to use some Actionscript... Attach it to a bar and make it so that the bar moves a movieclip in the opposite direction to the bar, and a relative distance to the bar itself.
Looks like you're after something like this or this.