That would indicate that index.php is not set as default page in the webserver.
For upgrade instructions, please see the manual:
Upgrading vBulletin Also check if there are specific instructions inside the package for the release you are installing.
PS The site that deals with support on default vBulletin is the official company website:
www.vbulletin.com.
PPS If you are not too experienced in daeling with vBulletin i recommend setting up a test copy of your site to test any work before performing it on hte life site.