Program Files -- Steam -- steam apps -- Age of Empires III -- bin -- Campaign
Then from the editor select a civilization, then Reset HC and then press Set HC, and select Chomps 3, then 2011 Br, 2012 Po, 2013 Sp, or 2014 Ge.
Thus: Campaign -> HCs -> Chomps 3 -> File
The first time you select to load a homecity, it will fail to load, just as with music files or with AI loader standard. If you press OK afterwards, then load your scenario and Set HC again, it will work. Note that Computers cannot use custom decks, unless you enable it from the AIHeader or AIMain.xs.