In order to use any function you have to instantiate the class(es) that contain the function. If it's a static function then you can call it directly. Make sure the classes in your includes list and the page can actually find the class. You can also use the constants used in the header.inc.php file to make sure you are targetting the correct paths.
You can then call them this way:
$yourvar = object -> function(parameter1,parameter2);
echo $yourvar;
If you don't understand the basics of classes do a Google search, there are tons of resources that explain it.
Updating my BoonexNerd.net site.