Did you upload the forums directory from our downloaded code to whatever server path your vbulletin exists at? For instance if your forums are in your web root you would have uploaded the contents of the forums directory of files from us to your web root.
a 404 error meens file not found.. that meens that when you do http://www.yoursite.com/admincp/vbgallery_install.php the file is not there!!!
the content of "forums/admincp/" in the package goes to your admincp folder.. (where ever you have put it!!!)
the content of the "forums/includes" folder goes to your vbulletin includes folder...
And other completely different thing..
IMHO i think your forumspath should start with a slash if you are on linux..
so not 'home/sitename/public_html'
(this should though NOT give you a 404 error...) 404 meens that the file was not found where you called it.