There's now 2 miniAtmegatron tutorials on the Tutorials page.
The first shows you how to assembly it and the second shows you how to MIDI-hack your Arduino Uno. If you bought a MIDI-hacked Arduino Uno with your miniAtmegatron, there is no need to watch the second one. The MIDI-hack vid is for Windows users only. A Mac one will be uploaded next week (it's a different process for Mac).