Upgrade plugin ‘premium.php’ missing

I have configured the upgrade plugin. All went well, even payment went well.

Only after choosing “return to ….” in PayPal tries to load the following page, and premium.php does *not* exist, resulting in a URL not found error:

http://fotomodo.d-dig.com/wp-admin/premium.php?page=upgrades_credits&updated=true&updatedmsg=Transaction+Complete%21

Where do I get premium.php?