Installing an SSL certificate is not a straight forward task.
It's not just a case of copying it to a directory on your server.
You will need to have direct access to run terminal commands on your server.
You will then also need to configure Apache to listen for requests on port 443.
If none of this makes sense, you will need to speak to your hosts, who will be able to provide more detailed support.
It really isn't a XenForo thing.