1 line Plug in Not working

[nascimbeni]

I added a chat on my vbulletin
This variable $vbulletin->session->vars['sessionchat'];
should show the number of users in the chat

I added simple 1 line plug in
$chat_users=$vbulletin->session->vars['sessionchat'];

then in navbar I simply have added
<td>Users in Chat: $chat_users</td>

It does not work - Which hook location should I use ?

( Maybe I do not need a plug in )
I am still confused in the hook / plug in system