After payment an email is sent to your inbox with a link on how to activate/validate your license. Click on that link and fill in all the information fields like name, email address username and password then click on Submit.
When submitted you will be redirected to Velocidrone website to login, if not click link below. When logged in find a link called "'My Licenses'" to download the full version of the simulator.
https://www.velocidrone.com/account/my_licenses