Answered Load your custom classes in MODx Revolution

class Foo {
    function HelloWorld() {
       echo 'Hello World';
    }
}

$foo = new Foo; 

$xpdo->loadClass('Foo','classes/');

if (!$xpdo->loadClass('Foo','classes/',true,true)) {
    die('Could not load class Foo!');
}
$Box = new Foo();

$message =  $Box->HelloWorld();
echo $message;

[[$_Foo]]

class Foo {
    function HelloWorld() {
       $message = 'Hello World';
	return $message;
    }
}

$modx->loadClass('Foo','classes/');
 
if (!$modx->loadClass('Foo','classes/',true,true)) {
    die('Could not load class Foo!');
}

$modx->loadClass('Foo',MODX_CORE_PATH.'classes/');
  
if (!$modx->loadClass('Foo',MODX_CORE_PATH.'classes/',true,true)) {
    die('Could not load class Foo!');
}

[[$_Foo]]

[[Foo]]

[[!Foo]]