OK, so you have done the basics:
- gotten MySQL and PHP and Apache (if applicable) working OK;
- created a empty MySQL database to hold your forum, and given it a name;
- created a MySQl user to own and operate that database too, if needed.
- then decided on the location for your punBB files in the folder system on your server (usually something like /home/username/mysitename/www/mypunbbforum
Then you've followed the punBB docs:
Installation
Copy/upload all contents of the directory upload/ into the directory where you want to run your forums.
Run install.php from the forum root directory (e.g. open http://mydomain/myforums/install.php in your browser). Follow the instructions.
Then you have setup your "Database type", "Database server hostname" "Database name" "Table prefix" "Administrator username" "Administrator password 1" "Administrator's e-mail" "Base URL" as instructed by install.php.
And then you have created a new file called 'config.php' which you have placed in the root of the directory you want to run punbb in -eg /home/username/mysitename/www/mypunbbforum.
So you now have a website folder hierarchy that looks like this:
/www - the standard root of your Apache (or whatever) website,
and within that your punbb forum folders:
-/mypunbbforum - the root of your forum, with PHP files like register.php, viewforum.php and config.php there
and the following folders:
-/mypunbbforum/cache/
-/mypunbbforum/img/
-/mypunbbforum/include/
-/mypunbbforum/lang/
-/mypunbbforum/style/
That's all that is usually technically necessary to kick punBB into life. Do the basic config operations, then upload the files into the right place.
Please note that there is NO folder named 'upload' in the example above - that's just meant to be the name of the folder you upload your files FROM, into a folder like /www/mypunbbforum/ on your server.
If you have successfully done all the technical steps above, I suspect your problem may be as simple as having uploaded all your files in the wrong place. Maybe check that step again.
In particular, ditch the 'upload' folder and just unzip or move the main punBB files into your root punbb folder. In particular your config.php file. That's where punBB is looking for it.