s2Member uses WordPress’ functions to know if the user is logged in or not. It’s WordPress the one that manages the login session of the users, not s2Member. If WP tells s2Member the person is not logged in, there’s nothing we can do. :/
Try this in a page that is not restricted: add a conditional to check if the person is logged in or out. You can even do this without s2Member active, to check how the login session is working. Install a PHP execution plugin, and add this to a page using only the text editor (not the visual one):
<?php
if (is_user_logged_in())
echo "You're logged in";
else
echo "You're not logged in";
?>
Knowledge Base » ezPHP Plugin for WordPress®
http://codex.wordpress.org/Function_Reference/is_user_logged_in
Check that it’s working correctly, then make sure you are logged in, link to that page from the PowerPoint file and try the link. Did the login session work correctly? Can you test it with another application like OpenOffice?