So what's the actual code you're using? You gave us the post, but you didn't show us the code you're using. This works perfectly fine:
<?php
include('SSI.php');
ssi_login('http://www.site.com/demo_index.php');
echo '<br />' . $context['user']['avatar']['image'];
echo '<br />';
ssi_logout('http://www.site.com/demo_index.php');
Now, if you go to localhost/forum, you need to see the mainpage of the form, not the installer. It would seem you're missing a table which seems to say you either (A) didn't go through the installer or (😎 don't have a full forum up and running.
Also note that SMF 2.0 is still beta software and is subject to bugs; although, I did get this example working with 2.0b3.1.