The good scenario suggesting to you to take part in battle between British and Americans. Both that and others landed ashore in northern part of a map and that is logical, took opposite positions. The task of Americans for whom you also should play — to destroy British, and thus to save the small town from occupation.
Copy files in the directory my documents \\My Games \\Age of Empires 3 \\Scenario. Launch game and select the desirable scenario from the Single Player — Custom Scenario menu.