SSI.php is included in SMF. To use it, you have to use the following code on the main page of your website:

<?php 
require_once('/absolute/path/to/SSI.php');
ssi_recentTopics();
?>

Please see ssi_examples.php, located on your forum's root directory, for more examples.

For more information on SSI.php, take a look here.

If you are not using a separate webpage for your homepage and people land on your forum index, you can show recent posts on the default forum index as well by going to Admin > current theme > Number of recent posts to display on board index