Hi
If you go to the resources/snippets page. Click "new snippet" and paste the following:
<?php
include_once($modx->config['base_path'] . '/forum/index.php');
return ;
?>
Name it the way you want (eg forumcall) and place a call on the page you want. [!forumcall!].
Above snippet code presumes the SMF is in a directory called forum. (
forum/index.php')

This can be read in step three on the beginning of this thread. Best.