Hi, first of all, welcome to the MODx community
To get you started, what you will need to do is create a snippet that includes the SMF SSI.php and then you can utilise the SSI function, ssi_RecentTopics(). The code below illustrates the basic construct of the snippet:
/**
* Snippet: Show SMF recent posts
*
*/
$basePath = $modx->getConfig['base_path'];
//-- CHANGE THIS TO THE FOLDER PATH OF YOUR SMF INSTALL
$smfPath = 'forums/';
include_once $basePath.$smfPath.'SSI.php';
return ssi_RecentTopics();
Check out the function definition in SSI.php for the additional options you can set when calling the ssi_RecentTopics function. The above is only the basic output.
Hope that helps,
Garry