On your server, you need to find the wpconfig file. If you ftp into it, it should be on the level with the index file. You'll need to copy it to your computer, open it with a text editor like notepad, add the line, save it, and then recopy it over to the server.
If you have cpanel access, use the file manager to go to the domain on the same level as the index file and you can edit the wpconfig file on the server directly.