You shouldn’t be editing xml files on the Player.
All you need to do is install it, open the Player Settings application and enter the CMS URL and key. The URL will be the same you use to access the login page - eg http://server/xibo and the key will be the one in your CMS Settings.
You should be able to enter that same URL in to the browser on that PC and see the CMS login page. If you can’t, you need to fix that before trying to configure the Player. As you say, it would most likely be a firewall or other access control, but because it’s a custom install it’s very hard for us to guide you as we don’t know what you’ve got setup or how.