I just started playing with ofxMaxim there are a few tutorials online and example code on github. I'm not sure if it has a midi to frequency conversion built in but you can write a function. Here is a guide scroll down to the bottom for code example. http://subsynth.sourceforge.net/midinote2freq.html.
Here is the youtube ofxMaxim tutorials.
Also there are a few other options. I still use supercollider but for quick ios game sound effects or something I would probably use the built in addon. Cheers!