To use this add-on, you simply need to load the helper:

$less = Loader::helper("less", "less");

Then, simply link to your .less file as you would a stylesheet:

print $less->link('styles.css.less');

That's it - you should find that a CSS file gets included.

Note that you need to ensure that the top-level CSS directory is writeable by the web server for this add-on to work.