Cannot add a hook without an associated plugin namespace

Hello Folks,
I would like to show Item Relationships on an exhibit item show, the URL looks something like this:
http://.../exhibits/show/themename/sectionname/item/itemnumber

I can get the words: "Hello World" on to the page (of some exhibits) by editing this file:
/exhibitbuilder/views/public/exhibits/item.php with the following:
echo "Hello World";

So I have begun with plugins

Using the Plugin Use Cases/writing your first Omeka Plugin example I can get the "hello_world_print_message" to show on public (default) item shows. The plugin does not show from inside the exhibit so I try putting the add_plugin_hook('public_append_to_items_show', 'hello_world_print_message');
line into:
/exhibitbuilder/views/public/exhibits/item.php

but I get the error
- Cannot add a hook without an associated plugin namespace

1) Where have I gone wrong? What documents should I read please?

and or

2) How can I get Item Relationships to show on an item show from within and exhibit please?

Thanks
Nick Kirlew
www.ntlexhibit.nt.gov.au

Modifying Exhibit Builder - items/show

And in our case it is overwrite show.php with show.php