I'm not sure if it is a plug in or mod. But if you would like to turn it on
Administration panel (administracja) -> Settings(Ustawienia) -> users on-line(Użytkownicy online)

But if you would like to get this plug in on your private forum (not pun.pl) I suggest you to contact with administration of pun.pl forum.

Ps. I hope you would understand what I'm talking about. I'm not good in English. :D