Gather all parts - littleBits synth kit, Arduino kit, powered USB hub, iPad, Apple Camera Connection Kit, MIDI Bridge iOS app, USB MIDI controller
Assemble a littleBits synth, test it using the littleBits keyboard, make sure it works.
Install the ARCore USB MIDI library into your PC/Mac/Linux computer's Arduino installation. Program the littleBits Arduino with the code attached to this project. The ARCore library is here: https://github.com/rkistner/arcore
Insure that your Arduino can be programmed, and you should have the "Arduino Leonardo (arcore iPad compatible" board selected. If you don't see that board option, you haven't got arcore installed into the right directory tree.
Install your programmed Arduino into your littleBits synth, probably in place of the littlebits keyboard. The power module plugs into the left of the Arduino, and the synth plugs into the D5 output of the Arduino. The D5 output should be switched to its "analog" setting.
Connect all USB devices to your powered USB hub:
Arduino via its included USB cable
iPad via Camera Connection Kit
USB MIDI controller keyboard
Start the MIDI Bridge app on your iPad, and connect your USB keyboard to the Arduino within MIDI Bridge. When you play keys on your keyboard on MIDI Channel #1, the line between your keyboard and the Arduino within MIDI Bridge should flash, and your synthesizer should play.
If desired, change the MIDI channel by playing a note between C1 and B1. C1=Ch#1, C#1=Ch#2, D1=Ch#3, etc... all the way up to B1=Ch#15.