I don't program in Xcode but when I need to have for example Processing send data to MaxMSP (and vice versa) I use OSC (OpenSoundControl). One of the advantages of OSC is that it enables communication wihtin one machine as well as between different machines on a network/the internet. You could build that into your game. But for sure there's other ways to do this, especially if the game and Max are running on the same machine.
Another option is the Unity game engine. It supports
Mac and windows, has a script based editor, and can
output data to max via osc. Unity might be easier to
get up and going on game ideas as opposed to wrestling
with c++ source code.