Are you able to install the default theme twenty fifteen? This is usually a server issue which may need you to enable DEBUG in wp-config.php to see the exact error and many a times, it may require you to look at the server error log to know why the Internal Server Error is occurring. Pls post a temp login if you are not able to see this. Thanks
The purchase code that you have is for Fusion theme and it is not for a rev slider plugin.
We have already bundled rev slider plugin zip file with theme , you just need to import the same and just activate the plugin just like you did for other plugin activation.
You can find the rev slider plugin zip file in frameworks/plugin folder .
Just ignore the activation code notice of rev slider and go ahead .