First you will need to open the Audio MIDI Setup application found in Applications/Utilities on your Mac.
Once it is open, you will need to navigate to the Window menu item and select Show MIDI Studio.
Double click the Bluetooth icon in MIDI Studio.
A list of Bluetooth devices should appear. Click the connect button to connect to the Adafruit Bluefruit LE device.
Next, use your favorite MIDI software synth to connect to the Adafruit Bluefruit LE device. If you don't have any MIDI software synths installed, a good option to start with is SimpleSynth. Once you have selected the Bluefruit, you should be able to press buttons on the UNTZ to send messages to your software synth.