If it's just showing 'Index of /' that's suggesting the web root is empty.
I think it's either the files are in the wrong place or the server's not configured properly.
What happens when you just place an index.html file in the web root which outputs "Hello World" ?
Make sure the MODX index.php file is in the public_html directory. The core, setup, and connectors directories should be there as well.
