I believe the video on this website will answer your question. http://www.sustainablesolutions.com/video.php
Click on Caroma Dual Flush Maintenance under General Maintenance near the bottom of the page.
Interiors of Texas, November 2007
remove lid on your toilet sistern
adjust the screw on the arm of the ball float to suit.
test hight of water by looking at the thin groove mark at the back of the cistern
if that does not work you may have to buy a new arm and float