I understand your frustation and I'm doing all my best to help you. The installation is fully tested (there are more than 1000 customers working with it), so I need to know if the server is reporting anything during installation. An "Internal Server Error" can be caused by several reasons, and usually you see what's happening in the "error.log" file of the server.
Can you provide me administrative credentials to access your backend and also your cpanel? If so use the email from which you receive forum notifications.
I have done some tests and finally I was able to install the extension using the "Install from url" tab. I don't know why you can't install it using the "Upload package file"; maybe there is a security measure (mod_security).