It sounds like you have too much text for the size of the text box and you're looking for a way to scroll the existing text upwards so that you can fit more text in at the bottom? What do you want to happen with the rest of the text above, do you want to be able to scroll back up and see it?
Edit: If you just want to keep adding text at the bottom use:
text.textheight>text.height
Trigger Once - Set Text Height to self.height+(the height of a single line of your text in pixels)