Sorry, I failed to add the other bit of code you need !!
The last paragraph should have read :
…and then on your page template add this function where you want it.
<?php if(is_front_page() ) { display_latest_topic () ; } ?>
I’ve wrapped it in an if is_front_page statement so it only shows on the frontpage. You night need to use is_home instead depending on how your blog is set up.
If you want it in the body, you should put it within the content div, either before or after the loop depending if you want it at the of or bottom of the page. You may need to play a bit !
If any of that isn’t clear, do come back, but let me know what theme you’re using as well