@Pilou that warning is not really important, you can safely disregard it.
It's really strange that it does not work for you. I've just tested it on my installation and it works just fine out of the box.
You can troubleshoot this further by editing the hook code, and inserting return statement such as
php
return '-------------------HERE--------------';
on different places. Start with the very beginning of the hook, so you know that the hook is is actually called by your theme. You should see this in your product page (don't forget to clear cache in Administration > Performace after every edit):
If you see this, then edit the hook again, and move it further down the code, just before the first return '';. Clear cache, and check product page again. If you see the same result, it means that hook is prematurely terminated because one of the condition in preceding if statement. If you don't see anything, move it before the next return ''; statement.
That should give you the idea what and where goes wrong