Open forums/SSI.php and find the code entries to use in a php include. I have Invision on here, so I'll show a couple of code examples with it, but it should be close to the the same with the Simple Machines Forum. The command string to put in your website header may be as complex as
Code:
<?php
include('http://www.darkproto.net/forums/ssi.php?m=posts&a=categorytopics&category=1&order=lastpost&start=0&show=5');
?>
or as simple as
Code:
<?php include('http://www.darkproto.net/forums/ssi.php?m=stats&a=members'); ?>
Those are copied directly from my header.php file. You'll just have to look around in the file and see what options you have to play with.