1) Go to 'Themes/default' directory in your SMF installation, as shown:

2) Open BoardIndex.template.php and find the following text (near line 352):

// Assuming there ARE users online... each user in users_online has an id, username, name, group, href, and link.
if (!empty($context['users_online']))
echo '
', $txt[140], ':<br />', implode(', ', $context['list_users_online']);

echo '
<br />', $context['show_stats'] && !$settings['show_sp1_info'] ? '
<a href="' . $scripturl . '?action=stats">' . $txt['smf223'] . '</a>' : '', '
</span>
</td></tr>';

Place your cursor right after the last semicolon (line 363), and paste the following section of code:

// flashchat users
echo '
<tr>
<td class="catbg" colspan="2">FlashChat Users</td>
</tr>
<tr>
<td class="windowbg">&nbsp;</td>
<td class="windowbg2" width="100%"><iframe frameborder="0" height="100" src="', $GLOBALS['boardurl'], '/chat/info_embedded.php" style="width: 100%; margin: 0; padding: 0; border: 0; overflow: hidden;" scrolling="no" border="0" allowtransparency="true"></iframe></td>
</tr>';

3) Save the changes, and point your browser to the index page of your SMF forum. You should see something like this: