Glad it worked out in the end!
If anyone else struggles with this:
After finalising the purchase they'll send you an activation code.
This is where you enter the code and set a password (which you will be asked to enter when you log-in).
As Winnie said, you will then receive an account-id that you should save somewhere/write down, because additionally to the password, you'll need it to access the player.
On a Windows PC or MAC you don't need to install anything. The player works inside your browser of choice, as long as you have flash installed and activated.
This is the direct link to the log-in, where you have to enter your account-id and your password.
The player then opens automatically.
The 3-day period starts as soon as you use the activation code. You can make the purchase in advance and then use the activation code later when you want to start watching.