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('path/to/SSI.php'); ssi_recentTopics(); ssi_topPoster(); ssi_logOnline(); ?>
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