How do I wire my CD74HC4067 multiplexer for my 61-note matrixed MIDI keyboard?

It would, and you would have 256 crosspoints available, but as I explained, if you have the diodes anyway, a 74HC4017 has ten outputs and together with the 74HC4067, you have 160 crosspoints and the 4017 is easier to code.

All that said, a single 4067 and four Arduino pins makes a 4 by 16 = 64 matrix.