Welcome to the forums datapusher
Have you tried the path Thomas or Bob have suggested? If so and it did not work please open a ticket with the help desk and they will give you the correct path to use. Link to help desk above or in my signature.
You're close. Your require statement needs to know where to look for the include. Try this:
>require("/home/your-cpanel-id/public_html/db.php");
This assumes db.php is in your public_html folder.
You can use phpAdmin to examine and change your database. It's located in cPanel. I wouldn't go making changes if you are unsure on what you are doing.