There should be something like this in your forumdisplay template (AdminCP > Styles & Templates > Style Manager > (From right panel) Edit Templates > Forum Display Templates > forumdisplay).
Look for:
Code:
<if condition="$show['forumslist']">
<!-- sub-forum list -->
...LOTS OF CODE IN HERE...
<!-- / sub-forum list -->
</if>
Cut all of that section out (Ctrl+X - Windows)
Then find:
Code:
...PASTE IN CODE HERE...
$ad_location[ad_forumdisplay_afterthreads]
<!-- Active Users in this Forum (and sub-forums) and Moderators -->
Paste in the code as indicated above.