Hi!
Finally someone from Gamersgate answered this.
It seems you have to first buy the RoP pre-order and then will be offered a 75% price reduction when you proceed to buy WIA.
Here is what he posted at Paradox Forums
"Sorry if we are a bit unclear about this bonus. We are working to improve it and make it more clear.
How it works right now is that everyone that have bought Rise of Prussia (that is have it on their account, then when they go in and make a new purchase it will give you the 75% discount on Birth of America II - Wars in America. We are working on our system to make it so that you can buy both games in the same order and get the discount.
Sorry for the confusion.
__________________
Daniel (mistbite)
Partner & Product Manager
GamersGate.com"