Sorry for the late reply. The codes are hidden in the game. One of the codes I have listed in a post here. The rest are hinted at in the game. Sort of an easter egg for those who find them. :)
I'm also really glad that you like the game. I am always doing my best to keep the updates coming out at a steady pace with a huge focus on quality over haste. There is also a new update out now that I really hope that you enjoy. ^_^