I still dont understand what you are doing there.
You say, you have a working online installation of OpenCart, and (as far as I unterstand) you will run this installation locally on your PC?
How did you install OpenCart locally? Is it a file system backup from your server and copied onto to your PC? Or is it a "real" installation, like you did for your online server?
And how did you install your online databases (including the database for OpenCart)? What did you do to have your online databases on your local PC?
You also said, that you adapt config.php (or similar) and you changed the line for "HTTP_SERVER". In a couple of forums i read, that this is NOT sufficient, there *should* be another configuration file in the admin folder, which also contains similar entries and these have to be adapted as well. How about that, did you do and can you show us, what is meant by that? As this is NOT an OpenCart board, i cannot give OpenCart support, you have to help us instead in these questions.
What and how did you configure in Opencart for the MySQL Databases Access? You somewhere must provide a database name, a server name, a MySQL User and its passwort. What did you specify and how did you do it?