Please login into your Xerver account first or
Please create an new free account if you don't have a Xerver premium account: http://xerver.co/?op=registration
You will receive a premium key like this 999y78we7udzBBBBB that you can exchange here: http://xerver.co/?op=my_account
Enter Voucher code / Premium key in field with that description "Apply Premium Key:" and click "Apply" button