Man, you drive me crazy !!
JUST DO WHAT I TOLD BEFORE - open the file that currently is your
index.php and don'T care about anything else. Add the code there, and everything will be fine.
No matter whatever you do - this code has to be inside
index.php - no matter what its content is ...
forum.php is the former, original vB-file "index.php" <-- fine, don't touch!
index.php is any other file from a portal or something <-- edit THIS ONE as its name is "index.php" now
The instructions in AdminCP -> Arcade -> Information told you this, I told you in this thread - why not simply do it ? Don't think too much, just follow the instructions...