3dfly - There are quite a few changes from versions 2.0 - 3.1. One of the biggest is that most all the templates were changed so that it would mesh better with vBulletin. If you look at the screen shots in the first post you can see that there's not
much of a difference... If you would like to keep the templates the way they are you can still upgrade, it just makes it a little harder since you will have to edit a few of the templates to apply any changes. If you would like to change to the new template style then upgrading should be a little easier. Either way, let me know if you wish to keep the old style or switch to the new one and I will tell you what you need to do to upgrade.
PKRWUD - What version of vBulletin are you running? I don't see why you would get an error if you edited the admin/functions.php file correctly, but it's possible it could be something to do with the version of vBulletin you're running. As for the path, it should be the path to your forum folder, not the url. Try this instead - /home/pkrwud/public_html/vbulletin