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

<?php include('http://www.example.com/SSI.php?ssi_function=recentTopics'); ?>
<?php include('http://www.example.com/SSI.php?ssi_function=topPoster');?>
<?php include('http://www.example.com/SSI.php?ssi_function=whosOnline');?>

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