Okay, so a custom map is an XML file and should be saved to the directory:
C:\Users\[username]\Games\Age of Empires 3 DE\[weirdnumber]\RandMaps
A custom scenario (single or multiplayer) is an AGE3YSCN file and should be saved to the directory:
C:\Users\[username]\Games\Age of Empires 3 DE\[weird number]\Scenario
In order to choose a custom map you’ll click on Skirmish and then choose the custom map in game setup. In order to load a custom scenario, click the Load button on the main menu.