MyBB, $forumdir is invalid!

I'm sorry for interrupting, but I use MyBB and EzIRC.
In the readme.txt there is this line;
Edit chat.php with the correct forumpath (search for $forumdir, at the top).
I used it like this;
$forumdir = "/dir"; // change this if you want the script to work outside your forums..
and this...
$forumdir = ""; // change this if you want the script to work outside your forums..
AND this...
$forumdir = ""; // change this if you want the script to work outside your forums..

(dir = the forum dir.)
It just doesnt works, it allways give me this line if I navigate to the chat.php:
$forumdir is invalid!

What do I do wrong? What to type in exactly? :/