How to display related categories?

There are no options for that actually. Here is a way though:
www.hikashop.com/en/forum/4-how-to/42759...duct-categories.html

For the title, you can add a line like that:
echo 'Related Categories</br>';

Linking to the categories themselves is a bit more tricky though:

global $Itemid;
foreach($this->categories as $category){
$link = hikashop_completeLink('category&task=listing&cid='. $category->category_id.'&Itemid='.$Itemid);
echo '<a href="'.$link.'">'.$category->category_name.'</a><br/>';
}