You find the mage under the marketplace tab next to the oracle at the top on android, for pc it is listed on the left side of the screen. Not sure about ios. Next time post in this is the Q and A thread that is stickied in forums please
You have to have castle level 3 which you can get after you explore 15 lands(I think may be off) once you explored and upgraded, go to the Marketplace, it is found there. If on PC it has it's on tab on the bottom left of your screen.