Keep in mind, this doesn't work for everyone and tends to not work. In-game mod launcher - Open FOC, go to Options, click Mods, and click the mod you want to load.
Set Launch Options - Get the ID of the mod (in the mod's URL, there should be numbers like 1129810972, those numbers are the ID), go to Steam library, right click Empire at War, click Properties, and you should see a launch options text box, in it, type STEAMMOD=.
