Login for the first time
After you registered for the online service,
you'll be asked for your login data the first time you connect to the online service.
The form will look like this:
Note: you only need a License Key on PC - you have received the license key in your e-mail, when you purchased the game.
The Username and Password are the ones you chose when you registered.
Once you do this the first time, the game will remember the data for you, and you won't need to go through this process again.
If you want to change your login credentials, you can do so from the settings menu on the home screen (gears icon -> erase login data).