A few examples to load Nodes (forums, groups, downloads or gallery categories, themes, etc.):
$form->add( new \IPS\Helpers\Form\Node( 'setting_name', \IPS\Settings::i()->
setting_name ? \IPS\Settings::i()->setting_name : 0, FALSE, array( 'class' => 'IPS\forums\Forum', 'zeroVal' => 'use_cat_forum' ), NULL, NULL, NULL, 'setting_name' ) );
The important here is the 'class' => 'IPS\forums\Forum'. You can use other elements:
'class' => '\IPS\Theme'
'class' => 'IPS\Application'
'class' => 'IPS\gallery\Album'
'class' => 'IPS\gallery\Category'
'class' => 'IPS\downloads\Category'