Place this where you wish the online code to be displayed:

[start:whos_online]
<b>Users Online: [total_online]</b><br />
[start:users_online]
<a href="[member_url][whos_online_id]">[whos_online_username]</a><br />
[stop:users_online]
[stop:whos_online]

This is just a basic online code for anyone that needs it.