Hi. A couple of ideas. First is, you shouldn't need to edit the wp-config file. If you're just starting out and haven't gone too far down the path, I'd suggest trying a couple of things.
1) You might want to first try to get a clean install of WP going. If you're using Plesk, instructions for doing that are here. How to install WordPress on your hosting account
Clean slate is the best place to start, if you have the option to do that. (That said, would also encourage you to check out the P.S. below.)
2) Once that's up and running using one of the pre-installed themes, then install your Themeforest theme. Make sure that's running. Again, you should be able to do pretty much everything through the various UIs, and shouldn't need to muck with the wp-config file at this stage.
3) If you then want to modify the Themeforest theme, I STRONGLY recommend doing it via a child theme. It'll save you hours and hours of hassle down the line. Here's how to make a child theme.
Hope this helps a bit.
-Chris
P.S. Is there a reason you chose Plesk? There are Managed WordPress options available that can handle a whole bunch of things (e.g. staging sites, automatic backups, automatic upgrades, etc.) automagically for you.