I would recommend you read the how to use this forum sticky post to find out how to ask a question here, it is all a bit rambling and your words are hard to make sense of.
The post will also tell you to post a link to all non standard libiries.
If this one is for making the Arduino look like a HID USB MIDI device it is indeed the wrong processor. You need an Arduino with a 32u4 processor like the Leonardo, Micro or Micro Pro. You could also use one of the Teensey range.
Or you need to use a program like Hairless in your Mac to convert normal serial data into a virtual MIDI device. Then you can still use your nano.