Use the arrow keys on the keyboard.
i used Sidetrack on g3 and g4 ibooks for years with no problems.
seems to be no longer in development, but the old version should work fine.
since you can't buy it, you'll just get a nag screen now and then that you can close.
with the settings, you can set a scroll zone, hot corners, whatever. works well on 10.3 and 10.4, never tried it on anything newer. best of luck.
EDIT: here's Ver 1.5.1 as a .dmg for free