These are directions for hacking the wp-forum plugin to work with wordpress mu. The need for the hack derives from problems trying to change sub-blog forum skins. Using the admin interface, you can change the main blog skin from the default setting, but not any of the sub-blogs. Basically anytime a sub-blog tries to change the skin of their forum, it not only fails to do so, reverting back to the default skin, but actually changes the forum skin on the main blog instead. The current work around that I’m using allows EITHER all sub-blogs and the main blog to have the same forum skin (other than the default skin), OR the main blog forum to have a different skin from uniform skin that all the sub-blog forums have. [Note that the later of these two options DOES NOT STOP THE PROBLEM of users with admin privileges on sub-blogs from changing the main blog’s forum skin accidentally. ]
The hack can be done prior to uploading the plugin, or by directly editing the files live.  Basically you are going to replace the default skin. The plugin comes preloaded with three skins to choose from aside from default: sweet, tiger, and web2. Delete the default skin folder. Open up the folder of the skin you want to use and edit the style.css file, changing line #2 where it says “Skin name: whatever” to “Skin name: Default”. Then change the name of the folder to Default and you’re done.  The skin folders are located at /wp-content/plugins/wp-forum/skins/
If you want the main blog’s forum skin to be different from the uniform sub-blog forum skin, select the skin you want in the admin interface. If you want all the forums skins to be the same, and thereby disable sub-blogs ability to accidentally change the main blog’s forum skin, delete all the skin folders except for the new default one.
The original plugin, written by Fredrik Fahlstad, can be found at fahlstad.se/wp-plugins/wp-forum/