I hate reverse engineering code - often find that if it doesn't make sense, it's because there is an error - lot's of wasted time
So, I'll just ask what does MIDI.read give you? i.e. what does it return?
The key played in some form? An int that maps 144 keys ?
If so, I'd suggest that the numbers in float_notes.h are the translation from 'key' to stepper frequency.
You'll not that the fraction between '0' and '12' is 2 ... i.e. 12 notes per octave ...
If so, it's very likely something along the lines of:
stepperFrequency = float notes[MIDI.read];
then use 'steperFrequency' to send the command to whatever you're using to drive the stepper...