if the index.php is meant to be in the forum directory.. move it back there.
then, download the .htaccess file and open it with notepad .. there will most likely be a single entry in there, but if there is more than one just look for the line that reads "DirectoryIndex filename.*".. change that line to "DirectoryIndex /forum_dir/index.php"
(if that line isn't there.. add it)
if you want to be clever
you can also put in a redirect for all the requests coming in to the old default file.. by adding the line:
"Redirect oldfile.html http://yoursite.com/forum_dir/index.php"
obviously the values i've used as examples will need to be changed eg: forum_dir, oldfile.html.. etc