You can scroll the text left and right by clicking the arrows on the Smartie preview window, but I don't think there is currently any way to use the keyboard to make lines scroll on demand.
If you un-check the "Don't scroll this line" checkbox next to the screen settings in the Smartie setup window, Smartie will automatically scroll the line if it is longer than the display.
If you don't like the built-in scrolling, look at the Scroll, ScrollPad, Vertical Scroll and Scroll'n'Bounce (and there are others) plugins, which do more flexible scrolling.