I open up the endorphin in 2017 just in case it's about the time problem and it says register now but when I click it loads up a page that no longer exists.
I've checked and it appears that NaturalMotion endorphin is still available on the official website, but there is nothing related to registration which means that you'll have to contact the developers to ask them about the error you've encountered.
Access the following link then fill the form: http://www.naturalmotion.com/contacts
Note: There is always the possibility that your version isn't supported anymore and a new version is required instead.