My exact code is as follows for the "Admin log in as user" mod. I didn't want to click their profile everytime to use it, so I have it so I have a tiny icon in there postbit.
Code:
<!-- Start Admin Log In As User -->
<if condition="in_array($bbuserinfo['usergroupid'], array(6))">
<a href="http://www.4x4mecca.com/forum/index.php?$session[sessionurl]u=$post[userid]&admin_log_in_as_user=$post[userid]"><img class="inlineimg" src="http://www.4x4mecca.com/forum/images/kirsch/buttons/lastpost.gif" alt="Log in as $post[username]" border="0" /></a>
</if>
<!-- End Admin Log In As User -->