Hit the ENTER key and key in the code

Use a text editor to edit the "proto.xml" file in the "data" directory in the game folder. Note: You may first need to uncheck the "Read Only" attribute of that file's properties in Windows Explorer. Search for the text "CrateOf". You will find CrateofFood, CrateofCoin, CrateofWood, CrateofFoodLarge, CrafteofCoinLarge, and CrateofWoodLarge. For all these entries, find the tag "InitialResourceCount" and change the count to whatever desired. For example, the normal crates each have "100.000" of the resources. You can change the "100" to "99999.000", etc.

Open the sp_yourcityname_homecity.xml file in the My Documents\My Games\Age of Empires 3\Savegame (dont forget to make a back up first) folder, search for <level>.
You will see something like this <level>xx</level>, put in 99 for the xx, save the file, go into the game and play a skirmish. Now you are level 99 !

Open the sp_yourcityname_homecity.xml file located in \My Documents\My Games\Age of Empires 3\Savegame (dont forget to make a backup of this file) find this line <skillpoints>.

The line should look like this <skillpoints>xx</skillpoints>, where x is the number of skillpoints you currently have. Change the xx to a high number such as 999, save the file and start Age of Empires 3. Go into a skirmish game, go to manage your city and you will now have 999 unlock points to unlock various things for your city.

There are several secret units that don't, at the moment of this writing, have a code to access them during games. Despite this, they can still be accessed in the editor through a simple process. First, place a unit on the map, then select the "Replace Unit" command under "Objects." In the drop down menu, select Fluffy, Flying Purple Tapir, Learicorn, George Crushington, Lazerbear, or Monster Truck A and then click replace unit.